Our DBus API (and mostly every other API in existence) define an
area as the top-left corner and width/height; glReadPixels on the
other hand uses the bottom-left corner, so we have to transform the
coordinates before passing them to GL.
https://bugzilla.gnome.org/show_bug.cgi?id=670979
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
