Level background music stops playing after starting next level

[davidjoffe]

This is a new bug, or rather, regression.

(Thanks for the report from this thread https://github.com/davidjoffe/dave_gnukem/issues/132 for arch Linux)

I just reproduced on latest Windows builds if using cheats to advance to next level.

The only recent changes I'm aware of in the sound code are moving from Mix_FadeInMusic to Mix_FadeInMusicPos ... not sure if it could have anything to do with that (and on Windows specifically the only other recent changes is using 'sdl2-mixer-ext' instead of sdl2-mixer' in vcpkg but that shouldn't affect Linux builds at all.)

This is relatively high priority (time permitting) for next stable (1.0.4 ideally)