core: Rename BitmapData->BitmapRawData, wrap it in new BitmapData

[adrian17]

This does make bitmap_data.rs uglier (it now has three copies of fn height() and similar methods), but on the other hand:

• The new names are clearer
• Most two-phase construction and GcCell handling are abstracted away:

// before
let bitmap_data = BitmapData::new_with_pixels(...);
let bitmapdata_wrapper = BitmapDataWrapper::new(GcCell::new(mc, bitmap_data));
let bitmapdata_avm2 = BitmapDataObject::from_bitmap_data_internal(..., bitmapdata_wrapper);
// after
let bitmapdata = BitmapData::new_with_pixels(mc, ...);
let bitmapdata_avm2 = BitmapDataObject::from_bitmap_data_internal(..., bitmapdata);

• It's a nice middle step towards future BitmapData refactors (including possibly data sharing), possibly without having to touch all these places again.