Implemented A2_NORMALIZE, A2_XFADE and A2_REVMIX

Normalization, crossfading (triangular window + 50% offset overlap-add)
and reverse mixing (mixing with a reversed copy of the wave) are now
implemented by a2_WaveUpload() and the stream API. (First a2_Flush()
only; it doesn't work when modifying prepared waves!)

Closes #32.