Raptor: add support for _PIC images

Includes test code.
Update test code to handle recent palette changes
Make tests use strings instead of fixed-length buffers

This stops displaying random data in the test failure messages if something
doesn't generate enough data to fill up to the expected length.
Test code: supply palette if needed when creating new tilesets
Add constants for image masks to improve code readability
Bit more tileset-fat documentation
Change zero-length tile manipulation result to separate variable

This is needed because the Vinyl tileset code produces a different (but
correct) result because of the order the actions are performed in.
Add support for Vinyl Goddess From Mars tilesets
Had the transparent/opaque constants around the wrong way
gameimg: Don't set palette unless caps includes CanSetPalette
VGFM: Add support for .scr images