perf: add a ShellGlobal property to call glFinish and log the time

It's useful to know how long frames are taking to render on the GPU.
This is impossible to measure in the normal case because frames may
parallelize with previous frames, but by calling glFinish() at the
end of the frame, we can create a (somewhat artificial, but useful)
environment where we have a meaningful timestamp for the frame
finishing drawing.

https://bugzilla.gnome.org/show_bug.cgi?id=732350