Fix rendertarget rendering (OpenGL)

This is a fix to make rendertarget work as expected in OpenGL. This is a followup to #1399.  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.

Also changed the behavior for the default  colorbuffer after being displayed (discarded instead of preserved).