Implement image masks to handle transparency in images.

This makes a whole bootload of images work properly.

LBX doesn't translate nicely to PNG when there are transparent pixels.
Therefore, lbximg will use 256-colour paletted PNG when there are no such
pixels, and 32-bit RGBA PNG when there is at least one such pixel.  This choice
is mainly because working with 32-bit RGBA PNGs takes forever on my laptop.

Apparently the simtex logo image contains some transparent pixels, thus the
regression test has been suitably updated.