Added a call to TextureCollcetion.Dirty() when changing render targets in OpenGL versions.
Merge pull request #1760 from bwfox/PR1758

Fix for issue #1758 - Drawing SpriteFonts to multiple RenderTarget2Ds consecutively leads to all but the first render target missing the text (WindowsGL)
Merge pull request #84 from mono/develop

Merge back from develop