Fix 3d transform primitives where the task offset is non-zero.

Fixes #2133.

webrender/res/prim_shared.glsl

@@ -648,7 +648,7 @@ VertexInfo write_transform_vertex(RectWithSize local_segment_rect,
     // We also want that to apply to any interpolators. However, we
     // want a constant Z across the primitive, since we're using it
     // for draw ordering - so scale by the W coord to ensure this.
-    vec4 final_pos = vec4(world_pos.xy * uDevicePixelRatio + task_offset,
+    vec4 final_pos = vec4((device_pos + task_offset) * world_pos.w,
                           z * world_pos.w,
                           world_pos.w);
     gl_Position = uTransform * final_pos;

wrench/reftests/transforms/content-offset.yaml

@@ -0,0 +1,16 @@
+---
+root:
+  items:
+    -
+      type: "stacking-context"
+      perspective: 1000
+      items:
+        -
+          type: "stacking-context"
+          transform: rotate-x(45) translate(100, 100, 0)
+          "transform-style": "preserve-3d"
+          items:
+              -
+                bounds: [0, 0, 200, 200]
+                type: rect
+                color: red

wrench/reftests/transforms/reftest.list

@@ -5,6 +5,8 @@
 # but fails when all the tests are run
 fuzzy(1,2) == rotated-image.yaml rotated-image.png
 == singular.yaml singular-ref.yaml
-fuzzy(1,82) == perspective.yaml perspective.png
+fuzzy(1,88) == perspective.yaml perspective.png
 == prim-suite.yaml prim-suite.png
 == segments-bug.yaml segments-bug-ref.yaml
+== content-offset.yaml content-offset.png
+