Fix some OpenGLES compile issues

Switch over to using regular FBO calls in RenderWindow
They are supported by ES 2.0 and are already being used
in vtkProjectedTetrahedra and they seem to compile on our
systems. Probably due to glew as it is not officially in
OpenGL 2.1. They are core in 3.0 but we check for the
extension to be safe.

Change-Id: Iffbb9ee4c08b97a7b9147eac768c1272cc54c0fc
Merge topic 'ios_fixes' into master

22dd7aee Fix some OpenGLES compile issues