* clutter/clutter-stage.c (clutter_stage_read_pixels): Use
OpenGL's coordinate system for the arguments to glReadPixels (so
that y zero is the bottom of the window). Use clutter_redraw
instead of clutter_stage_paint to ensure the right GL context is
selected. Set some of the glPixelStore parameters that might have
been changed by Cogl.
* tests/test-stage-read-pixels.c: Replace with a different test
that gets a sub-region of the stage around the cursor.
Add a note to the clutter_stage_read_pixels doc that the alpha channel
isn't guaranteed to contain sensible data
* tests/test-stage-read-pixels.c: (update_snapshot):
Overwrite the alpha data when using clutter_stage_read_pixels; fixes
this test on non-nvidia drivers
Correct documentation about accepted pixel formats
* tests/test-stage-read-pixels.c: (main):
Don't try to free a ClutterTexture after main, fixes segfault on quit
