1) Fix signature of _mpd_qpow_uint(): contrary to the comment base is constant.

1) Fix signature of _mpd_qpow_uint(): contrary to the comment base is constant.

2) Abort the loop for all specials, not only infinity.

3) Make the function more general and distinguish between zero clamping
   and folding down the exponent. The latter case is currently handled
   by setting context->clamp to 0 before calling the function.