Fix opengl rendertargets

This is a fix to make rendertarget work as expected in OpenGL. Here is the gist of it:

- There is no need to create one FBO per rendertarget; we can simply swap attachments when we call GraphicsDevice.SetRenderTargets; also this paves the way to multiple rendertargets support
- The default color buffer needs to be reset before frame swap (iOSGameView.Present) to ensure the one that might be created in the RenderTarget constructor  doesn't interfere.

Tested on Infinite Flight (iOS), we use rendertargets quite extensively.

Not sure why so many files got included in this pull request; let me know if this is a problem.