Refactor of how render target bindings are stored.
Added _currentRenderTargetCount.
_currentRenderTargetBindings is a fixed sized array and never null.
GetRenderTargets() now returns copy of array.
ApplyRenderTargets() now copies the incoming binding array elements.
Cleaned up a few unnessasary changes.
A few small cleanups.
Removed unnessasary GL.DeleteRenderbuffers() code.
Tried to make the error checking for FBO a little clearer.
We now correctly detach depth/stencil from the frame buffer.
Merge pull request #1666 from SickheadGames/rtfix

GraphicsDevice RenderTarget Fixes