Fixed exception when using Song class before MediaManager is initialised.
Added shutdown of media manager when game exits.
Update MediaManagerState.cs

Removed thread safety checks from media manager, as it should only be started or stopped from the UI thread anyway.

Caller should be responsible for ensuring this.
Resolving Conflict.
Fixing WINGL build.
Fixing WINRT build.
Merge branch 'develop' of https://github.com/SimonPStevens/MonoGame into SimonPStevens-develop

Conflicts:
	MonoGame.Framework/Media/MediaPlayer.cs
Merge pull request #1820 from totallyeviljake/PR_1509_UPDATE

#1509 Updated to resolve conflict - verified by @borrillis
Fix for Android's Mouse.GetState().
Merge branch 'develop' into MultiWindowMouseInput

Conflicts:
	MonoGame.Framework/Windows/WinFormsGameWindow.cs
Merge pull request #1749 from SickheadGames/MultiWindowMouseInput

Multi-Window Mouse Input
Merge branch 'develop' of git://github.com/mono/MonoGame into monogame-sdl2
SDL2 buildfix for last upstream merge.