Currently, all three render backends hold on texture-related
resources indefinitely (`register_bitmap` pushes to a `Vec`,
and never removes anything). As a result, the resources used
by the render backend (which may include GPU memory) will grow
over time, even if the corresponding `BitmapData` has been deallocated.
This commit adds a new `unregister_bitmap` method, which is called from
`BitmapData.dispose`. All render backs are changed to now use an
`FnvHashMap<BitmapHandle, _>` instead of a `Vec`, allowing us to
remove individual entries.
Currently, we only call `unregister_bitmap in response to
`BitmapData.dispose` - when `BitmapData` is freed by the
garbage collector, `unregister_bitmap` is *not* called.
This will be addressed in a future PR.
a79aa08f08