The screen grabber was a workaround for an extremely slow path in Mesa
when reading back pixel data from the frame buffer. It was using pixel
buffer objects by directly calling into GL to hit a fast blit path in
Intel's driver. This should no longer be necessary with the latest
Mesa because the normal read pixels path now has a fast path to just
memcpy the data. Using PBOs in that case just adds an extra
indirection because the data is read into an intermediate buffer and
then copied back out again.
We want to be able to remove the dependency on linking against libGL
directly from Gnome Shell because that will not work if Cogl is
actually using GLES. Also libGL includes GLX which means gnome-shell
ends up with a hard dependency on Xlib which hinders the goal of
getting Gnome Shell to be a Wayland compositor.
https://bugs.freedesktop.org/show_bug.cgi?id=46631https://bugzilla.gnome.org/show_bug.cgi?id=685915
For the Intel drivers, using glReadPixels() to read into client-memory
directly from the frame buffer is much slower than creating a pixel
buffer, copying into that, and then mapping that for reading. On other
drivers, the two approaches are likely to be similar in speed. Create
a ShellScreenGrabber abstraction that uses pixel buffers if available.
Use that for screenshots and screen recording.
https://bugzilla.gnome.org/show_bug.cgi?id=669065
