When drawing the graphics window, we flush it twice: once to draw
the geometry, and another time to draw the UI overlay (toolbar,
selection marquee, and FPS counter). Calling glFinish() each time
is (on most platforms) just pointlessly slow, but on macOS Catalina,
without offscreen rendering, it causes the toolbar to flicker.
Instead of calling glFinish() twice per frame in that case, call
glFlush() twice and then glFinish() once we really are done.
This is currently necessary to get repeatable results when exporting
assemblies as a part of a batch process, since the mesh geometry in
imported files is not regenerated for export.
