Add support for prescaled UV in vertex decoder JIT

Prescaled UV lets us avoid a very common shader uniform upload, the texture scale/offset, by baking the texture scale/offset into the vertices as we decode them. Prescaled UV can considerably help performance in games that really abuse the tex scale/offset to effectively increase the precision of its 8-bit texture coordinates, for example, by changing it every few triangle strips.

If you're using vertex caching the benefit of this change will probably not be very obvious but it's here for completeness anyway.