re-initialising the context was causing severe performance degradation (hangs,
CPU consumption & RAM) in the snap version (at least) when KDE QuickStyle was
used.
The composition works as follow:
* Both the interface and the video are rendered in an offscreen window (using
X11 composite extension)
* We register to damage event (x11 damage extension) to get notified when the
content of the offscreen video window change. When we receive a damage event
we ask the rendering part of the composition to refresh
* The interface is rendered in the offscreen surface using a RenderControl, when
the interface do render, the composition is asked to refresh
* A dedicated thread is spawned to do the rendering, upon a refresh event it
will take the pictures from the offscreen surface and blend them into the
actual window using X11 render extension. Using a separated thread from Qt
ensure that the rendering of the video will not be stalled if Qt thread is
busy.
* The damage events are listened on a separate X11 connection and on a separate
thread than Qt main thread (here the rendering thread). This allows to receive
theses events independently from Qt (in case the Qt thread is stalled). Note
that it is not possible to peek in qt X11 event queue from a non gui thread as
the QX11Info::peekEventQueue is no longer thread safe since 5.12.
fixes: #25627, #22155
Fatih Uzunoglu
Pierre Lamot
Benjamin Arnaud