Optimize texture unswizzling by walking pointers.

Also fix rowWidth = 1, there's no way that was correct, it wasn't even
increasing the address it was reading from.
Optimize common cases for unswizzled indexed tex.
Swap buffers after unswizzling, it's faster.
Merge pull request #1277 from unknownbrackets/perf2

More optimizations to texture loading (swizzled)