Transparent blits are not correctly supported

[PluMGMK]

The transparent blit code taken from the Video 7 driver checks against the background colour byte-by-byte, which is only suitable for 256-colour modes. It needs to be adjusted to check pixel-by-pixel, which is one, two, three or four bytes at a time, depending on the current mode.

Not sure how important this is, because I get the impression that most stuff under Windows 3.1 doesn't use transparent blits at all…