Vertex decoder JIT

This speeds up vertex decoding by precompiling the vertex decoders to native ARM or x86.

When vertex cache is on, there isn't that much difference as most vertices are cached and not re-decoded every frame, but even then it might reduce stutters and so on. With vertex cache off, this makes a noticable speed difference.

Thanks @unknownbrackets for filling in the blanks (16-bit color decoding) and finding and fixing some bugs.