GL: Swap R/B in A1R5G5B5 correctly.

Fixes Terraria, thanks @raven02.  How did I miss this, it was not passing
the test after all...
GL: Fix D1R5G5B5 texture R/B components as well.
Merge pull request #243 from unknownbrackets/texformats

Swap R/B in a few other texture formats