prim_shared: Fix clip mask perspective interpolation.

This was really hard to track down, the comment there really convinced me that
the code was correct, so I ended up looking at everything else before that...

This fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1494453.

webrender/res/prim_shared.glsl

@@ -241,8 +241,7 @@ float do_clip() {
         return 1.0;
     }
     // anything outside of the mask is considered transparent
-    //Note: we assume gl_FragCoord.w == interpolated(1 / vClipMaskUv.w)
-    vec2 mask_uv = vClipMaskUv.xy * gl_FragCoord.w;
+    vec2 mask_uv = vClipMaskUv.xy / vClipMaskUv.w;
     bvec2 left = lessThanEqual(vClipMaskUvBounds.xy, mask_uv); // inclusive
     bvec2 right = greaterThan(vClipMaskUvBounds.zw, mask_uv); // non-inclusive
     // bail out if the pixel is outside the valid bounds

wrench/reftests/transforms/perspective-clip-1.yaml

@@ -0,0 +1,25 @@
+---
+root:
+  items:
+    -
+      type: stacking-context
+      bounds: [0, 0, 0, 0]
+      perspective: [1, 0, 0, 0, 0, 1, 0, 0, -300, -250, 1, -0.5, 0, 0, 0, 1]
+      items:
+        -
+          bounds: [0, 0, 0, 0]
+          type: stacking-context
+          transform: [10, 0, 0, 0, 0, 10, 0, 0, 0, 0, 1, 0, -5382, -3222, -18, 1]
+          items:
+            -
+              bounds: [0, 0, 100, 100]
+              type: rect
+              color: red
+            -
+              bounds: [0, 0, 100, 100]
+              type: rect
+              color: green
+              complex-clip:
+                rect: [0, 0, 100, 100]
+                radius: 16
+

wrench/reftests/transforms/perspective-shadow.yaml

@@ -0,0 +1,23 @@
+---
+root:
+  items:
+    -
+      type: stacking-context
+      bounds: [0, 0, 0, 0]
+      transform: [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 139, 0, 1]
+      perspective: [1, 0, 0, 0, 0, 1, 0, 0, -299, -250, 1, -0.5, 0, 0, 0, 1]
+      items:
+        -
+          bounds: [0, 0, 0, 0]
+          type: stacking-context
+          transform: [10, 0, 0, 0, 0, 10, 0, 0, 0, 0, 1, 0, -5382, -3222, -18, 1]
+          items:
+            -
+              type: box-shadow
+              bounds: [548, 200, 100, 100]
+              color: 0 0 0 0.7020
+              blur-radius: 20
+            -
+              bounds: [548, 200, 100, 100]
+              type: rect
+              color: 0 255 0 1.0000

wrench/reftests/transforms/reftest.list

@@ -23,5 +23,7 @@ platform(linux,mac) == perspective-mask.yaml perspective-mask.png
 rotate-clip.yaml rotate-clip-ref.yaml
 clip-translate.yaml clip-translate-ref.yaml
 platform(linux,mac) == perspective-clip.yaml perspective-clip.png
+platform(linux,mac) == perspective-clip-1.yaml perspective-clip-1.png
+platform(linux,mac) == perspective-shadow.yaml perspective-shadow.png
 == complex-preserve-3d.yaml blank.yaml
 platform(linux,mac) fuzzy(9,348) == perspective-border-radius.yaml perspective-border-radius.png