vo_gpu: reduce the --alpha=blend-tiles checkerboard intensity

This was sort of grating by default and made it really hard to actually
read e.g. text on top of a transparent background. I decided to approach
the problem from both directions, making the whites darker and the grays
lighter. This brings it closer to the dynamic range of e.g. the
wikipedia transparent svg preview.

video/out/gpu/video.c

@@ -2767,7 +2767,7 @@ static void pass_draw_to_screen(struct gl_video *p, struct ra_fbo fbo)
             // Draw checkerboard pattern to indicate transparency
             GLSLF("// transparency checkerboard\n");
             GLSL(bvec2 tile = lessThan(fract(gl_FragCoord.xy * 1.0/32.0), vec2(0.5));)
-            GLSL(vec3 background = vec3(tile.x == tile.y ? 1.0 : 0.75);)
+            GLSL(vec3 background = vec3(tile.x == tile.y ? 0.93 : 0.87);)
             GLSL(color.rgb = mix(background, color.rgb, color.a);)
         } else if (p->opts.alpha_mode == ALPHA_BLEND) {
             // Blend into background color (usually black)