Bug 1893956 - Ensure consistent rounding for draw compositor tile and clip rects. r=gfx-reviewers,nical

In bug 1894106 we ensured that draw compositor tiles' tile rects and
clip rects were both calculated on the CPU, to avoid visible seams
whilst low-quality pinch zooming. However, the clip_rect's calculation
uses CompositeState::get_device_rect(), which rounds the rect, whereas
the newly added tile rect calculation used a raw call to
ScaleOffset::map_rect(), which does not. This means that seams still
sometimes flicker whilst zooming. This patch fixes this by using
CompositeState::get_device_rect() for the tile rect calculation as
well, ensuring consistent rounding.

Differential Revision: https://phabricator.services.mozilla.com/D222641

Author: jamienicol

webrender/src/renderer/mod.rs

@@ -3158,8 +3158,8 @@ impl Renderer {
             let tile = &composite_state.tiles[item.key];
 
             let clip_rect = item.rectangle;
+            let tile_rect = composite_state.get_device_rect(&tile.local_rect, tile.transform_index);
             let transform = composite_state.get_device_transform(tile.transform_index);
-            let tile_rect = transform.map_rect(&tile.local_rect);
             let flip = (transform.scale.x < 0.0, transform.scale.y < 0.0);
 
             // Work out the draw params based on the tile surface