Bug 1679700 - Cast int parameter in get_yuv_offset_vector to float, to work around a shader compilation bug with integer equality comparisons on macOS Intel. r=jrmuizel

We've encountered similar bugs before, e.g.
https://github.com/servo/webrender/wiki/Driver-issues#3540---mac-glsl-compiler-bug-with-integer-comparison-take-two

We don't want to change this `if` to a `switch` because glsl-opt compiles it
down to an if statement anyway, just a more complicated one (which was triggering
other bugs on other platforms, see bug 1678924).

Differential Revision: https://phabricator.services.mozilla.com/D98208

gfx/wr/webrender/res/yuv.glsl

@@ -86,7 +86,8 @@ mat3 get_yuv_color_matrix(int color_space) {
 }
 
 vec3 get_yuv_offset_vector(int color_space) {
-    if (color_space == YUV_COLOR_SPACE_IDENTITY) {
+    // Float conversion to work around a macOS Intel shader compiler bug
+    if (float(color_space) == float(YUV_COLOR_SPACE_IDENTITY)) {
         return vec3(0.0, 0.0, 0.0);
     } else {
         return vec3(0.06275, 0.50196, 0.50196);