Bug 1690319 - Remove integer comparison workaround in fetch_clip_area. r=mstange

This removes the workaround from servo#2864

The root cause of that has been identified as bug 1690027.

Differential Revision: https://phabricator.services.mozilla.com/D103797

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/18835f3f39365c75abed1f7db9b8dcdcfaf38f1b

Author: jrmuizel

webrender/res/render_task.glsl

@@ -98,7 +98,7 @@ struct ClipArea {
 ClipArea fetch_clip_area(int index) {
     ClipArea area;
 
-    if (index >= CLIP_TASK_EMPTY) {
+    if (index == CLIP_TASK_EMPTY) {
         RectWithSize rect = RectWithSize(vec2(0.0), vec2(0.0));
 
         area.common_data = RenderTaskCommonData(rect, 0.0);