This case was added in 662c793
for use in vo_gpu_next as a visibility test before rendering a frame.
The OpenGL context doesn't have this so it just returns true.

This is done to avoid cluttering vo_gpu_next.c with more ifdeffery and context-specific code
when additional backends are added in the near future.
Eventually gpu_ctx is intended to take the place of ra_ctx to further separate gpu and gpu_next.

Wrapping the context is pretty straightforward. This is only complicated
by needing to account for the upside-down framebuffer in a few places.