By creating a pending gbm/EGL surface pair, only setting it on
swap-buffers, we would draw onto a buffer on the old surface, then swap
the buffer from the new surface, causing the first frame after a
hot-plug always having no content.
This was in the past not very noticable since some non-deterministic but
frequent side effect in gnome-shell caused hot-plugging to always render
two new frames, but after "Introduce regional stage rendering", this
side effect did not occur as often, thus making it more visible.
This commit updates the current gbm/EGL surface pair before painting a
frame, so that when the frame is painted, the surface with the correct
size is used and the buffer from correct surface is swapped.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
e891a8b628