Modify an existing opengl application to render to a PBO (and from there to a file)?

Modify an existing opengl application to render to a PBO (and from there
to a file)?

pI want to modify an existing OpenGL application to render to a PBO and
then read the PBO to generate an encoded video of what was originally
going to be rendered to the screen. Since performance is key, I cannot
stall the pipeline by doing glReadPixels from the backbuffer as I was
doing. I am wondering if there is a simple or straightforward way to
redirect everything rendered to the framebuffer and make it instead go to
the PBO. In other words, I don't care if it is not shown on the screen. As
a matter of fact, I would prefer if nothing is shown to the screen./p