Bug 1730710 - Propagate raster spaces in GPU process r=gw

This actually does two things. One: it lets child stacking contexts inherit out
of process raster spaces by checking the parent stacking context's raster space
inside WR itself, and Two: it mirrors the logic in get_raster_space_for_prim
which undoes the device_pixel_scale for pinch zooms in order to get a
consistent glyph rasterization scale. This second thing won't affect the test
page linked in the bug, but it will affect the performance of, say, animating a
whole browser's scale, which is useful for reasons.

Differential Revision: https://phabricator.services.mozilla.com/D125562

gfx/layers/wr/StackingContextHelper.cpp

@@ -133,8 +133,7 @@ StackingContextHelper::StackingContextHelper(
     : mBuilder(&aBuilder),
       mScale(1.0f, 1.0f),
       mDeferredTransformItem(aParams.mDeferredTransformItem),
-      mRasterizeLocally(aParams.mRasterizeLocally ||
-                        aParentSC.mRasterizeLocally) {
+      mRasterizeLocally(aParams.mRasterizeLocally) {
   MOZ_ASSERT(!aContainerItem || aContainerItem->CreatesStackingContextHelper());
 
   mOrigin = aParentSC.mOrigin + aBounds.TopLeft();

gfx/wr/webrender/src/prim_store/text_run.rs

@@ -425,7 +425,12 @@ impl TextRunPrimitive {
                 RasterSpace::Local(rounded_up / device_pixel_scale.0)
             }
         } else {
-            self.requested_raster_space
+            // Assume that if we have a RasterSpace::Local, it is frequently changing, in which
+            // case we want to undo the device-pixel scale, as we do above.
+            match self.requested_raster_space {
+                RasterSpace::Local(scale) => RasterSpace::Local(scale / device_pixel_scale.0),
+                RasterSpace::Screen => RasterSpace::Screen,
+            }
         }
     }
 

gfx/wr/webrender/src/scene_building.rs

@@ -1819,8 +1819,11 @@ impl<'a> SceneBuilder<'a> {
     ) -> StackingContextInfo {
         profile_scope!("push_stacking_context");
 
-        // Push current requested raster space on stack for prims to access
-        self.raster_space_stack.push(requested_raster_space);
+        let new_space = match self.raster_space_stack.last() {
+            Some(RasterSpace::Local(scale)) => RasterSpace::Local(*scale),
+            Some(RasterSpace::Screen) | None => requested_raster_space,
+        };
+        self.raster_space_stack.push(new_space);
 
         // Get the transform-style of the parent stacking context,
         // which determines if we *might* need to draw this on