[AD] Big patches for the mixer

["Ben Davis" <ben@xxxxxxxxxx>]

I have patches for mixer.c, allegro.h, allegro._tx and faq._tx. I believe
they are ready to be committed. Unfortunately, my post was rejected because
mixer.c.diff is too big! :-) Can I send them to someone privately to be
committed please?

The new mixer has an extra function:

void set_volume_per_voice(int scale);

This function is documented in the new allegro._tx. I've tested it
thoroughly using sine waves, since they sound like square waves when they
distort. It took a lot of tweaking, but it's now working for all
combinations I tested (including all sound qualities).

In addition, silent samples now continue playing (without wasting time
actually mixing), and loops are handled better when doing interpolated
mixing. I've tested the former, but not the latter. When sample->loop_end ==
sample->len, with quality = 2, it should now loop without clicking (I used
to have to add an extra sample at the end of my musical instrument WAVs to
stop them from clicking!). I'm pretty sure it'll work.

I've checked that the documentation compiles properly, and it does.

Ben Davis