Add support for libopenmpt.

[CicTec]

Hi,

SDL2 currently supports libxmp and libmodplug as MODs decoders.
I think it would be useful to add support for libopenmpt, first because it is the successor to libmodplug (which is no longer maintained), and second because it is used by OpenMPT, so those who want to use particular effects with this composition tracker could do so (in the absence of support in libxmp for example).

[arabek]

Lacks a bit of explanation, why it would be rational to switch to libopenmpt:

• libopenmpt is being actively developed (as part of OpenMPT) by a group of dedicated demoscene musicians
• ompt is being actively tested for corner cases in module playback routines tracked under Test Cases

[slouken]

Thanks for the suggestion! We're about to revamp the API for the SDL 3.0 release, but once that's done we can revisit this.