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.
You now have a weird-ass hidden global that will hinder any future parallelization attempts. (As well as still not getting rid of unset values properly.)