Fix for issue #1758 - Drawing SpriteFonts to multiple RenderTarget2Ds consecutively leads to all but the first render target missing the text (WindowsGL)

Added a call to Textures.Dirty() in GraphicsDevice.ApplyRenderTargets(...). 

Fixes a bug in OpenGL versions where drawing a texture to multiple render targets one after the other would only produce visible results in the first render target drawn to.

Issue: https://github.com/mono/MonoGame/issues/1758