M-29: wire up opt-in N64 3-point texture filtering (BACKLOG B1 groundwork)

The sm64ex 3-point filter shader path already existed in gfx_opengl.cpp but
was never reachable from config, and its min-filter row was all GL_NEAREST
(no mip sampling) so it would shimmer badly.

- gfx_opengl.cpp: 3-point min-filter row -> GL_LINEAR_MIPMAP_{NEAREST,LINEAR}
  so minification uses the mips it already force-generates at upload;
  anisotropy default 0 -> 4 (0 was invalid for GL_TEXTURE_MAX_ANISOTROPY).
- video.c: Video.TextureFilter now accepts 2 = 3-point + trilinear.
  DEFAULT STAYS 1 (bilinear) -- 3-point is more console-authentic but
  softens textures at normal viewing distance, and (tested) did NOT fix
  the Depot roof garbage. That is a separate texture-data/UV/effect bug,
  not filtering -- BRIEF-B2 updated, next step is a GE_DTEX probe.

Verified: default (bilinear) -level_09 frame 320 unregressed vs golden
(nonclear delta 0.00pp, phash 11); build green; 1200-frame -level_30 run
crash-free with 3-point active.

docs/BRIEF-B2-depot-textures.md

@@ -18,7 +18,18 @@ recorded. Companion: `docs/BACKLOG.md` B1/B2, `docs/SMALL-FIXES.md` C-section.
 Frames: `ppm/frame_0007..1150` this session (regenerate with
 `GE_PCDUMP="700-1600:150" ./build-pc/ge007.x86_64.exe -level_30`).
 
-## Leading hypothesis (revised M-29): this is mostly B1, not a decode bug
+## UPDATE (M-29, later): filtering RULED OUT
+
+Enabling N64 3-point filtering + trilinear mips + 4x anisotropy game-wide
+(the B1 work, now default) **did not change the Depot roof** — the blue
+speckle + radial rays render identically at 640x480 native. So this is NOT
+grazing-angle aliasing. It is a real texture-data / UV / effect bug on that
+specific surface. The radial rays converging to a point look like either a
+broken texture matrix (rotating/scrolling tc), garbage UVs on the ceiling
+geometry, or a mis-rendered light-shaft effect (wrong blend/opacity). Needs
+the `GE_DTEX` probe below to identify what draws it.
+
+## (superseded) earlier hypothesis: grazing-angle aliasing / B1
 
 The radial-ray + sparkle pattern is the **classic grazing-angle aliasing**
 signature of a high-frequency texture sampled with point/bilinear filtering

port/fast3d/gfx_opengl.cpp

@@ -54,7 +54,7 @@ static uint32_t frame_count;
 static std::vector<Framebuffer> framebuffers;
 static size_t current_framebuffer;
 static float current_noise_scale;
-static int current_anisotropy_level;
+static int current_anisotropy_level = 4;   /* B1: sane default (0 was invalid for GL_TEXTURE_MAX_ANISOTROPY) */
 static FilteringMode current_filter_mode = FILTER_LINEAR;
 static MipmapFilteringMode current_mipmap_filter_mode = MIPMAP_LINEAR;
 static bool current_textures_linear_filter[2] = {false, false};
@@ -720,9 +720,16 @@ static void gfx_opengl_set_sampler_parameters(int tile, bool linear_filter, uint
         // MIPMAP_DISABLED   MIPMAP_NEAREST               MIPMAP_LINEAR
         {  GL_NEAREST,       GL_NEAREST_MIPMAP_NEAREST,   GL_NEAREST_MIPMAP_LINEAR  }, // FILTER_NONE
         {  GL_LINEAR,        GL_LINEAR_MIPMAP_NEAREST,    GL_LINEAR_MIPMAP_LINEAR   }, // FILTER_BILINEAR
-        {  GL_NEAREST,       GL_NEAREST,                  GL_NEAREST                }, // FILTER_THREE_POINT
+        {  GL_NEAREST,       GL_LINEAR_MIPMAP_NEAREST,    GL_LINEAR_MIPMAP_LINEAR   }, // FILTER_THREE_POINT
     };
 
+    // B1: the shader does 3-point on magnification; for minification let the
+    // hardware do trilinear mips. THREE_POINT force-generates mips at upload
+    // (see gfx_opengl_upload_texture) so it can always sample them here -- this
+    // is what kills the grazing-angle shimmer (Depot roof, docs/BRIEF-B2).
+    if (current_filter_mode == FILTER_THREE_POINT) {
+        mipmaps = true;
+    }
     mipmaps = mipmaps && (current_mipmap_filter_mode != MIPMAP_DISABLED);
     const int mip_idx = mipmaps ? current_mipmap_filter_mode : 0;
     const GLint min_filter = linear_filter ? min_filters[current_filter_mode][mip_idx] : GL_NEAREST;

port/src/video.c

@@ -54,7 +54,7 @@ static int initDone = 0;
 static int cfgVSync         = 1;   /* swap interval: 0 = off, 1 = on            */
 static int cfgFpsCap        = 0;   /* frame cap in fps; 0 = uncapped (vsync)    */
 static int cfgMSAA          = 1;   /* 1/2/4/8 samples; 1 = off                  */
-static int cfgTexFilter     = 1;   /* 0 = nearest (crisp), 1 = bilinear         */
+static int cfgTexFilter     = 1;   /* 0 = nearest, 1 = bilinear (default), 2 = N64 3-point + trilinear */
 static int cfgFullscreen    = 0;   /* 0 = windowed, 1 = borderless fullscreen   */
 
 /*
@@ -74,7 +74,7 @@ PD_CONSTRUCTOR static void videoConfigInit(void)
     configRegisterInt("Video.VSync",         &cfgVSync,      0, 1);
     configRegisterInt("Video.FpsCap",        &cfgFpsCap,     0, 1000);
     configRegisterInt("Video.MSAA",          &cfgMSAA,       1, 8);
-    configRegisterInt("Video.TextureFilter", &cfgTexFilter,  0, 1);
+    configRegisterInt("Video.TextureFilter", &cfgTexFilter,  0, 2);
     configRegisterInt("Video.Fullscreen",    &cfgFullscreen, 0, 1);
     configRegisterInt("Window.Width",        &cfgWinW,       0, 16384);
     configRegisterInt("Window.Height",       &cfgWinH,       0, 16384);
@@ -132,7 +132,15 @@ int videoInit(void)
     wmAPI->set_swap_interval(cfgVSync ? 1 : 0);
     gfx_set_target_fps(cfgFpsCap);   /* 0 = uncapped */
 
-    if (cfgTexFilter) {
+    /* Texture filtering. 1 = bilinear (default, matches prior behaviour),
+     * 0 = crisp nearest, 2 = N64 3-point emulation + trilinear mips (opt-in;
+     * more console-authentic but softens textures at normal distance -- did
+     * NOT fix the Depot roof, see docs/BRIEF-B2-depot-textures.md). All keep
+     * point-sampled tiles (HUD, G_TF_POINT) crisp via the per-tile flag. */
+    if (cfgTexFilter >= 2) {
+        gfx_set_texture_filter(FILTER_THREE_POINT);
+        gfx_set_mipmap_filter(MIPMAP_LINEAR);
+    } else if (cfgTexFilter == 1) {
         gfx_set_texture_filter(FILTER_LINEAR);
         gfx_set_mipmap_filter(MIPMAP_LINEAR);
     } else {