glFinish is meant to ensure every aspect of the screen is fully-drawn
before continuing. This causes issues with certain chipsets (Intel
4-8k)
glFlush is more of a suggestion that CPU work is complete and the GPU
should complete its work in a "timely" fashion.
We add an additional glFinish call before getting a screenshot as this
will copy the actual data from the card buffer.
Fixes: lp:1775976
* https://bugs.launchpad.net/kicad/+bug/1775976
f6bd6b1360