We hit a pathological case in House
(https://github.com/ruffle-rs/ruffle/issues/15154),
where eagerly decoding bitmaps during preloading results in
over 10GB of ram being used.
With this PR, we store the compressed bitmap, and only decode it
each time we instantiate it. In order to support bitmap fills,
we store the decoded width/height and a lazily-initialized GPU handle
in `Character::Bitmap`
900a8407d6