bug#28695: 26.0.60; Rendering lag spikes caused by double-buffering on Linux

[martin rudalics]

> I was able to verify that reverting to the previous commit before
> double-buffering was added completely removed the problem. I've made two
> short screencasts, one before the double-buffer commit and the other
> after.

Unless this problem (and that of Bug#28652) get fixed, we should either
disable double buffering by default or add some special warning so users
do not have to spend their time investigating this issue.

martin