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.
Why will it not get rid of unset values? Also, if you're talking about running multiple vertex shaders in parallel, you could instead keep multiple OutputVertex objects around for that to work.

Anyway, it doesn't really matter too much, since I'm not the guy who's concerned with speed anyway :) Keeping the memset is probably better for now indeed.