Fix handling of world clip rects when APZ is occurring.
Fix handling of world clip rects when APZ is occurring.

During APZ, it's possible to receive a new display with effectively
the same content, but with display items at different local
positions.

Handling this with world clip rects, while not invalidating tiles
unnecessarily as they change relative to scrolling tiles is
quite subtle. This patch fixes these by maintaining the region
of the primitive in its primitive-relative space that affects
the tile (similar to how primitive interning stores rects as
primitive relative).

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender