Add lyxrc option to force the use of backing store

Add lyxrc option to force the use of backing store

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 c213eb7f75d53165f55e64f0149fec833e5b5b5e)
(cherry picked from commit 1ed9257c6dc57fc11990b86d5774ac9a8e70cfa6)