glimagesink last sample orientation

Submitted by Nicola @drakkan

Link to original bug (#797205)

Description

Created attachment 373762
test case

please take a look at the attached test app,

basically glvideoflip ! glimagesink displays the right image but if video flip orientation does not change the image size (for example GST_VIDEO_ORIENTATION_180) the snapshot taken from last-sample has a wrong orientation.

I discussed about this issue with ystreey00 today and it seems there isn't a simple solution

"""
it's one of those edge cases that I'm not sure exactly where to fix it
"""

I added a workaround in my application (a videoflip element in the snapshot pipeline) but since xvimagesink (and probably d3dvideosink too) works as expected this behaviour should be at least documented. Thanks!

Attachment 373762, "test case":
test-snap-flip.c