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.
This uses MultiTileLayer instead of SingleTileLayer for devices that don't have
gralloc support. This has the advantage of reduced memory usage due to less
wastage from power-of-two textures, and can be a performance boost due to slow
sub-image updates on some devices.
We only ever want to respond to the latest viewport sent, separate the
viewport change into its own event and remove all but the latest in the queue
when processing events.
