In OpenGL ES 3.0, the default framebuffer should use
GL_FRAMEBUFFER_DEFAULT instead of GL_RENDERBUFFER for
default buffer. I also added the Framebuffer::IsRenderbuffer()
utility function to make the transition easier.

Change-Id: I041e610738a4a656555ae7fa7ba41ba68b1fe9ea
Reviewed-on: https://swiftshader-review.googlesource.com/4763Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>