Fixes for Screen and Control Gui classes
FIX: #1 Screen.RemoveAll() does not reset _focusedControlIndex - _focusedControlIndex is not set to 0 in Screen.RemoveAll.
FIX: #2 Memory leak in Screen and Control PrepareRenderTarget - PrepareRenderTarget will now check if the rendertarget needs to
be disposed and recreated. It will only be recreated if it's width and height has changed.
Merge pull request #3 from hfarrow/master

Fixes I made for Issues #1 and #2 that I entered.
Merge remote-tracking branch 'origin/master' into dev