System3.x uses the following bit expansion when converting RGB565 to
RGB888:
RRRRR GGGGGG BBBBB => RRRRRRRR GGGGGGGG BBBBBBBB
12345 123456 12345 12345123 12345612 12345123
However, SDL's blit functions use a slightly different mapping, so
sometimes expanded color did not match the color key specified in the CX
command.
To avoid this, convert to RGB888 by ourselves when drawing 16-bit CGs.
kichikuou