handle non-vertex, non-table fog.

- Implemented: D3DSIO_SGN, LOOP, ENDLOOP, LOGP, LIT, DST, SINCOS
- Process instruction-based modifiers (function existed, it just
  wasn't being called)
- Add loop checking to register maps.
- Renamed "sng" to "sgn" for D3DSIO_SGN - it's not handled anywhere
  except for GLSL, so won't matter.

based on documentation by Mike McCormack.

as CoUnmarshalInterface may have advanced the stream, even if it failed.