MacCursor not handling black in color cursors

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MacCursor doesn't handle black in color cursors (crsr resources). Black pixels aren't drawn because they're treated as transparent.

This can be seen in the Run cursor in QFG1VGA-Mac. There are three black pixels, two of which are in the leg, and they're not drawn.

The Mac format allows a full 256 palette plus a separate transparency mask but the Cursor model requires one color to to be the transparency key. MacCursor is hard-coded to return 0xff as the transparency key but color 0xff is always defined as black in Mac. There is no code that attempts to deal with this and so incoming 0xff pixels aren't drawn.

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Fixed in e3c179e975f2c1420eabb8aa2accf26f0d897fed