Make it possible to use QPainter on a QGLWidgets from a thread

As OpenGL is - with few exceptions - reentrant and since it is possible to use OpenGL calls to render to a QGLWidget from a thread it should also be possible to use a QPainter on such a widget from within a thread.

However, the static caches in QOpenGlPaintEngine break reentrancy for that engine. Using TLS or mutexes for the caches would allow painting to GL widgets from a thread.