[Bug 748748] compositor: only paint background if actually needed

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=748748

--- Comment #2 from Tim-Philipp Müller <t.i.m at zen.co.uk> ---
I think this does have performance benefits, I was just looking at a case where
painting the background unnecessarily was line #2 in perf top, and taking up
significant cpu power.

If I read your patch correctly, this covers the case of one input filling out
the entire output, which I guess makes sense if compositor is used for
picture-in-picture or logo overlay or such.

Any ideas how we could easily detect the case of a matrix/arrangement of input
streams filling the complete output frame? (Easy case: N*M matrix, Harder case:
e.g. one big picture with a row of little ones at the bottom)

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