Force positions to scale by 128/32768 as psp does.

This makes everything use floats for positions.  On some hardware/drivers,
this may be faster.  On some it may be slower.  We'll need testing to see
the performance impact.

Fixes Final Fantasy 4's pos misalignments, and probably others (like
Tekken 5 I suspect.)
Refactor s8/s16 to float conversion.

It's pretty much all done the same way.  Note that normals have always
been a little inconsistent between 127/128 and 32767/32768.  But it's
probably not causing a major impact...
Merge pull request #6737 from unknownbrackets/pos-scale

Force positions to scale by 128/32768 as the psp does