prim_shared: Fix clip mask perspective interpolation.

webrender/res/prim_shared.glsl

@@ -121,14 +121,12 @@ VertexInfo write_vertex(RectWithSize instance_rect,
     vec4 world_pos = transform.m * vec4(clamped_local_pos, 0.0, 1.0);
 
     // Convert the world positions to device pixel space.
-    vec2 device_pos = world_pos.xy / world_pos.w * uDevicePixelRatio;
+    vec2 device_pos = world_pos.xy * uDevicePixelRatio;
 
     // Apply offsets for the render task to get correct screen location.
-    vec2 final_pos = device_pos + snap_offset -
-                     task.content_origin +
-                     task.common_data.task_rect.p0;
+    vec2 final_offset = snap_offset - task.content_origin + task.common_data.task_rect.p0;
 
-    gl_Position = uTransform * vec4(final_pos, z, 1.0);
+    gl_Position = uTransform * vec4(device_pos + final_offset * world_pos.w, z * world_pos.w, world_pos.w);
 
     VertexInfo vi = VertexInfo(
         clamped_local_pos,

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