Fix for camera and scissor tool in 'export' mode
Detail:
- If the source sprite has an old-style bit mask, solid pixels in the exported sprite's mask may have a different value to the source sprite's. For example 0xFF -> 0xFC.
- The OS seems to render such intermediate mask values inconsistently. Applicable GCOL commands have now replaced with OS_SetColour calls (taken from another fix elsewhere in the code that dates back to Acorn). Admin: See also https://www.riscosopen.org/forum/forums/4/topics/14698 Submission for the Paint bounty.