[vulkan] Comment out some unreachable code

When targeting spir-v, qfcc does do dead code removal yet, so the return
followed the previous return (with no intervening label) so caused the
shader to not pass validation.

Now the fragment shaders work (mostly: there's a problem with discard in
the sprite shader).

libs/video/renderer/vulkan/shader/bsp_sky.frag

@@ -78,7 +78,7 @@ sky_color (vec3 dir, float time)
 		vec4        c1 = sky_sheet (dir, time);
 		vec4        c2 = sky_box (dir, time);
 		return vec4 (mix (c2.rgb, c1.rgb, c1.a), max (c1.a, c2.a));
-		return vec4 (1, 0, 1, 1);
+		//return vec4 (1, 0, 1, 1);
 	}
 }