Rasterizer Optimizations

This PR includes several optimizations to the rasterizer, each found to give a small boost in performance on my test case. (First 25 frames of 3dscraft, note that this isn't GPU-bound ATM, so not the best representative of a performance increase.) Each commit message has the run time for the test as well as the delta from the previous commit on its commit message.
The majority of that article has examples for other kinds of formats, so I didn't think it really applied. (Even though yes, that's exactly what we're doing here, decoding textures.)