We already honor AA_ShareOpenGLContexts and pass in the QGuiApp's
context as the shareContext for the QRhi's QOpenGLContext. Extend this
to also allow specifying a QOpenGLContext in the init params struct.
A good example of this is the backingstore compositor that serves
QQuickWidget and co. If one wanted to implement that with (an
OpenGL-based) QRhi, instead of direct OpenGL calls, then the ability
to create a QRhi that uses a context that shares resources with a
given other context becomes essential.
Pick-to: 6.2
Change-Id: I6bc5ff8e803d467f8795197ac1f12fdc0f73bbd1
Reviewed-by: Christian Strømme <christian.stromme@qt.io>
afa0ce9045