MouseState.Button out of date

MonoGame stops processing mouse events when the game is not active, leading to the mouse buttons being 'out of date' when the game window becomes active again. 

Fix is to reset to current actual state on window activate.
So we have bigger problems here...  mouse state just doesn't work at all like XNA.  So i'm looking to make a bigger fix that will remove this work around and implement the correct XNA like behavior.  Funny that @thefiddler already enumerated the problems over in https://github.com/mono/MonoGame/issues/2464. :)