clutter-stage: fix crash caused by using deprecated api
clutter_set_viewport always operates on the draw fb, but mutter sometimes has an offscreen fb it uses instead.
This commit uses the non-deprecated, clutter_framebuffer_set_viewport function, which takes an explicit fb argument.
This is from a downstream bug https://bugzilla.redhat.com/show_bug.cgi?id=1508903 late last year that I forgot to upstream.
I didn't thoroughly investigate the fix, just guessed, and it seemed to work.