Since `initial_data` was removed from `Character::Bitmap` in #9143,
it now holds a single field. Move back to an unnamed field, which
aligns with the other `Character` enum variants.
Now that a `Bitmap` always stores a `BitmapData`, we can read the pixels
directly from the `BitmapData`, instead of duplicating them in an
`initial_data` field
We only called `get_bitmap_pixels` when creating a `BitmapData`
for an SWF-provided `Bitmap`. We now store the initial pixels
in `Character::Bitmap`, and use them to initialize a `BitmapData`
when needed.
This lets us simplify the wgpu backend, which no longer needs
to store a `Bitmap` object. In addition to saving space for
`BitmapData` objects that lack an SWF `Bitmap`, this will make
it easier to move data from `bitmap_registry` into `BitmapHandle`
itself.
Converts the Bitmap character to a proper display object. This can
be instantiated directly in a PlaceObject tag in SWFv9 movies,
compared to the previous versions which indirectly references
bitmaps from Shape tags.