Playing mono samples with libacm

[winterheart]

Hello!

We recently integrated libacm in our Descent 3 game project (https://github.com/DescentDevelopers/Descent3). It uses ACM audio files in both mono and stereo. After integration, we encountered a problem that samples with one channel were cut off during playback. We found that in this case acm_open_decoder() with force_chans = 0 forces two channels despite the correct ACM header.

Although we can do force_chans = 1/2, this requires separate processing of the ACM header, which is not optimal from the point of view of the logic of our code. Is it possible to remove this check in the code?

Proposed changes here.

[DanielGibson]

I have a better idea on how this can be handled in a way that keeps the existing behavior of libacm, but also allows telling it to trust the header.

See #3

[markokr]

Fixed by #3