Stop the media player before disposing.

[xanather]

This needs looking into as to why we need to do this in the first place

https://github.com/flibitijibibo/MonoGame/issues?direction=desc&sort=updated&state=open

[flibitijibibo]

See the accompanying issue. I honestly think something's on your end; I've got too many games using the MediaPlayer to think that they're somehow all missing something like this.

Verify that everything with the MediaPlayer is fixed first in XNA4, then get a full trace on every bug you find. I'm not pulling anything related to the MediaPlayer if it prevents XNA4 compliance on our end or the user's end, end of story.