Fix a number of bugs related to the tile cache.
Fix a number of bugs related to the tile cache.

* Fix panic where update_transforms would try to calculate transforms with invalid child/parent relationship.
* Fix extra invalidation where retained tiles didn't have the right tile size / raster fields in descriptors.
* Fix panic where primitive rect was incorrectly calculated outside the bounds of the tile cache.
* Fix invalidation of unused tiles that exist in the grid.
* Fix dirty rect calculation including unused tiles.

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