GL: opt-in PS1 dithering ([video] dithering = off|on|scaled)

The OpenGL renderer has always drawn shaded and texture-modulated pixels
at 8 bits per channel with no dither pattern (true colour). That stays the
default. 'on' applies the PS1 4x4 offset matrix and 5-bit truncation per
internal-resolution pixel; 'scaled' puts the pattern on the native grid,
so it reads like the console at any scale and the native VRAM is the same
at every internal resolution.

GP0(E1h) bit 9 reaches the backend through a new optional facade hook
(gr_set_dither) recorded for the render thread and Smooth motion redraws.
With the mode off nothing flushes and no uniform changes. Flat, raw and
other 5-bit-exact pixels are never touched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/include/gpu_render.h

@@ -61,6 +61,9 @@ void gr_set_precise_triangle(int enabled,
  * perspective_texturing). q[i] is the normalized 1/z homogeneous weight at
  * vertex i. enabled == 0 restores the PS1's affine UV interpolation. */
 void gr_set_perspective_triangle(int enabled, float q0, float q1, float q2);
+/* GP0(E1h) bit 9 (dither enable). Only a backend with [video] dithering
+ * implements it; the default renderers stay true colour. */
+void gr_set_dither(int enabled);
 /* PGXP depth for the NEXT triangle (docs/ENHANCEMENTS.md G1.14): the GTE SZ
  * of each vertex (1..65535), taken from validated dataflow shadows. Only a
  * backend with a PGXP depth buffer / perspective-correct colour uses it;
@@ -167,6 +170,7 @@ typedef struct GpuRenderBackend {
                                  int32_t x1, int32_t y1,
                                  int32_t x2, int32_t y2);
     void (*set_perspective_triangle)(int enabled, float q0, float q1, float q2);
+    void (*set_dither)(int enabled);   /* optional: GP0(E1h) bit 9 */
     void (*fill_rect)(int x, int y, int w, int h, uint16_t color);
     void (*copy_rect)(int src_x, int src_y, int dst_x, int dst_y, int w, int h);
     void (*draw_flat_triangle)(int x0, int y0, int x1, int y1, int x2, int y2,