mmdevice: allow to set the volume per player rather than per session

this patch aim to allow setting the volume per player when mmdevice backend is used

the original issue is that if you spawn multiple libvlc player in a program, calling libvlc_audio_set_volume will set the volume for all instances rather than only the given player. The feature remains optional as we certainly want to keep the current behavior for VLC as a player(same volume in the app and in windows mixer)

the patch is greatly inspired from the work from @tguillem here https://patches.videolan.org/patch/20354/