Add OpenMPT support

[SiegeLord]

No description provided.

[pedro-w]

How do you disable DUMB in Cmake? All the other codecs seem to have a WANT_XXX variable, but not DUMB.

[edit] got it, it's WANT_MODAUDIO

[pedro-w]

How are you testing this? I tried using ex_acodec with some .mod files I have (and know to be ok) but the error dialog said "Could not load sample from 'XXX.mod'!" (nor did it work with the DUMB renderer on released version of A5 so I guess it's ex_acodec ?)

[SiegeLord]

I compile it with both enabled, but hack it to use openmpt to load .xm files rather than DUMB. I generally use ex_stream_file with --loop or not. ex_stream_seek is also pretty good.

At the moment, I'm struggling to get looping to work, I don't really understand how openmpt handles looping (the only relevant APIs are openmpt_module_get_ctls with play.at_end and openmpt_module_set_repeat_count).