[RFC] Set volume straight after music is set playing

[mugmuggy]

Fixes #1978

Describe what the proposed change does

• Sets the volume after starting playing the track - SDL2_mixers midi volume function on windows uses a handle to the stream to set the volume which requires that Mix_PlayMusic is called to open that handle initially.

[lewri]

It looks like this fixes the first song playback. Unfortunately, however it isn't sent until music is played.
The volume of the opening music is still stuck sadly until you get to main menu (looks like video still not sending SDL volume update)

[mugmuggy]

I think we'd need a fix for 524 to actually get that to happen. Windows is remembering the last volume set, which due to 524 is the volume of the music in game from the last time its run. What you really need is that full system volume range is used and then the ratios apply. I have a hacky patch in mind for sdl2_mixer for the 524 issue, I just need to see how it integrates with the full behaviour once integrated.

[lewri]

Happy to proceed with this, but should probably make a new issue (or adjust 524) about the specific intro video bug.

[lewri]

@mugmuggy do you know if our changes to SDL2/Fluidsynth for Windows in 0.67 still require this PR?