direct mode renderer. I thought that reads were not much slower than
writes, so that the method only tripled the time for the whole function,
but I recently measured that video memory reads can be up to 53 times
slower than writes in tighter loops than here. Loop overheap here
reduces the multiplier to only 16-20 on Haswell.
Start cleaning up and fixing larger bugs in this function. Only replace
the 22-line removal loop by a 3-line one for now, since adjusting the
old loop would have required many palette calculations which are better
done in the DRAW_PIXEL() macro. This also fixes missing support for
depth 24, but only for removal.
Removal is currently sloppy at the right bottom corner. It sometimes
leaks border color into the text window. This is soon cleaned up by the
caller. The planar renderer has complications to clip at the corner.
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