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.
What advantage does this memset provide over making `ret` a static variable that is initialized to 0 once and then never again?