Quickdraw packs data as a series of codes that the application is supposed
to handle, but it does not define any order in which they might appear.
Since it's unfeasible to support *all* opcodes defined by the spec,
only handle well-known blocks containing video data and ignore any unknown
or unsupported ones.

Move palette loading and rle decoding to separate functions to resue them
in other blocks and drop format initialization in init since it can
support more formats than pal8.

Validate width and height.