The Director editor supports importing images from a variety of image
formats, which it then converts. There is also the option for linked
bitmap cast members (i.e. using image files on the disk), which have a
much more restricted subset of formats. As of Director 4, this list
includes Windows Bitmap files, and anything supported by the Apple
Picture Viewer (i.e. any possible encoding of PICT, and JPEG images).

File extensions are ignored, so we use some basic magic number checking to
determine which ImageDecoder class to use.

Fixes viewing the manifesto pages in DEVO Presents: Adventures of the
Smart Patrol.

When loading bitmap cast members from a file, Director will keep an
copy of what palette the image uses. However the order of colours in
the image's own palette can be completely different!

It appears that at load time, Director will remap all of the colours in
the image to match the order in the palette cast member. As such, we can
force a remapping for those external cast members using the existing
dither code.

Fixes the colours in Cat The Ripper.