This replaces the waving flag SEQ file with a PNG file.

Added support for color-keyed PNG files.  It's not perfect, I but I don't expect us to use more than one colour of transparency in our images for now.

Adds new images extracted from flag.seq):
flag.seq.0.png (US waving flag, with US palette)
flag.seq.1.png (USSR waving flag, with USSR palette)