LyX relies on a a backing store to draw when running under macOS or
Wayland, because Qt arbitrarily overwrites parts of the workarea
before we paint (and we paint only the parts that need to be painted).
However it seems that this is also necessary on X11 when the WM theme
is translucid. Since there is no way that I know of to detect this
situation, this patch adds a LyXRC setting to manually select this
drawing strategy.
Note that using a backing store is not always a good solution, since
this disables subpixel aliasing.
At this point there is no UI for the variable.
Fixes bug #12119
(cherry picked from commit c213eb7f75)
(cherry picked from commit 1ed9257c6d)
1def11d296