Instead of tying the tile-buffer in MultiTileLayer directly to the back-buffer
of the page, make sure rendering is always aligned to the tile grid and use
tiles on-demand. This makes better use of tiles when panning/zooming, and opens
up the route for further optimisations.
Alter GeckoSoftwareLayerClient.beginDrawing so that it can return a rectangle
of the buffer that it's interested in. Gecko then uses this to clip the dirty
region, which often saves on unnecessary drawing during flings.
This adds support for rendering to a tiled software buffer when using the
Java compositor. Tiles are assumed to be packed tight and stored sequentially,
by row.
