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