- Add InitInstruction to hw shader backend, and remove SetUnusedSource.

- Add InitInstruction to hw shader backend, and remove SetUnusedSource.
- NV30FP/NV40VP: Clear any fields before we OR new values into them
- NV40VP: It seems that it might be possible to write a result reg at the
    same time a temp is written. In InitInstruction, initialise OUT_DEST to
    OUT_DEST_TEMP so result regs don't get clobbered by default.