vo_opengl: draw transparency checkerboard after upscaling

This also draws it after color management etc. In a nutshell, this
change makes the transparency checkerboard independent of upscaling,
panning, cropping etc. It will always be the same apparent size and
position (relative to the window).

It will also be independent of the video colorspace and such things.
(Note: This might cause white imbalance issues if playing a file with a
white point that does not match the display, in absolute colorimetric
mode. But that's uncommon, especially in conjunction with transparent
image files, so it's not a primary concern here)

video/out/opengl/video.c

@@ -1730,10 +1730,6 @@ static void pass_convert_yuv(struct gl_video *p)
         GLSL(color.a = 1.0;)
     } else if (p->opts.alpha_mode == 2) { // blend against black
         GLSL(color = vec4(color.rgb * color.a, 1.0);)
-    } else if (p->opts.alpha_mode == 3) { // blend against tiles
-        GLSL(bvec2 tile = lessThan(fract(gl_FragCoord.xy / 32.0), vec2(0.5));)
-        GLSL(vec3 background = vec3(tile.x == tile.y ? 1.0 : 0.75);)
-        GLSL(color.rgb = color.rgb * color.a + background * (1.0 - color.a);)
     } else { // alpha present in image
         p->components = 4;
         GLSL(color = vec4(color.rgb * color.a, color.a);)
@@ -2171,8 +2167,18 @@ static void pass_draw_to_screen(struct gl_video *p, int fbo)
         GLSL(color.rgb = clamp(color.rgb, 0.0, 1.0);)
         GLSL(color.rgb = pow(color.rgb, vec3(user_gamma));)
     }
+
     pass_colormanage(p, p->image_params.primaries,
                      p->use_linear ? MP_CSP_TRC_LINEAR : p->image_params.gamma);
+
+    // Draw checkerboard pattern to indicate transparency
+    if (p->has_alpha && p->opts.alpha_mode == 3) {
+        GLSLF("// transparency checkerboard\n");
+        GLSL(bvec2 tile = lessThan(fract(gl_FragCoord.xy / 32.0), vec2(0.5));)
+        GLSL(vec3 background = vec3(tile.x == tile.y ? 1.0 : 0.75);)
+        GLSL(color.rgb = mix(background, color.rgb, color.a);)
+    }
+
     pass_dither(p);
     finish_pass_direct(p, fbo, p->vp_w, p->vp_h, &p->dst_rect);
 }