Prescale in the vertex dec jit. Needs debugging.
Prescale UV in vtx-dec-jit: Fix bugs, add ARM support
Add commented-out code to save XMM4/XMM5.

According to all calling convention manuals I can find, we don't really
need to preserve them. If they become problematic as mentioned, we can
activate this.
Merge pull request #4468 from hrydgard/vtxdec-prescale

Add support for prescaled UV in vertex decoder JIT