Is it possible to render to the backbuffer and a FBO at the same time?
The GL 2.1 spec doesn’t talk about FBOs, so I refered to the GL 3.0 spec.
p 256 says
Table 4.5: Arguments to DrawBuffer(s) and ReadBuffer when the
context is bound to a framebuffer object, and the buffers they indicate.
i in COLOR ATTACHMENTi may range from zero to the value of
MAX COLOR ATTACHMENTS - 1.