lbm: Fix Extra-Half-Brite usage. - #758
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If the caller loads an ILBM file with 64 colors, and either provided a colormap with <= 32 colors or set the Extra_Half-Brite flag, then duplicate the lower 32 entries to half-intensity versions in the upper half. This also initializes colormap to zero unconditionally, in case this logic would otherwise touch uninitialized stack memory. Now if there's a weird file that didn't provide all 32 colors, the original and half-brite ones will just be black instead of random data. https://en.wikipedia.org/wiki/ILBM#Extra_Half-Brite (Also an SDL_memset for an unrelated thing was changed to SDL_zero, since I was in here anyhow.)
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If the caller loads an ILBM file with 64 colors, and either provided a colormap with <= 32 colors or set the Extra_Half-Brite flag, then duplicate the lower 32 entries to half-intensity versions in the upper half.
This also initializes colormap to zero unconditionally, in case this logic would otherwise touch uninitialized stack memory. Now if there's a weird file that didn't provide all 32 colors, the original and half-brite ones will just be black instead of random data.
https://en.wikipedia.org/wiki/ILBM#Extra_Half-Brite
(Also an SDL_memset for an unrelated thing was changed to SDL_zero, since I was in here anyhow.)