Cleanup macro-mayhem

This converts the VERTBLEND macros to some clever templates. This code
should be much more maintainable.
Fill volatile buffers

This shaves off a memcpy and some smaller copies. Decent gains that scale
up quite well as more avatars are on screen.
SSE 4.1 Skinning

This appears to offer no benefit on my Wolfdale CPU, but I expect it will
be useful on more recent ones.