The driver used on the Odroid-XU3 does not like doing just glFlush()
before accessing the texture in another context. There is no guarantee
that glFlush() is enough to sync access to resources between contexts, so
start using glFinish() as the default, except on common desktop hw + iOS
where flush is enough and presumably more efficient.
To unify the code pathes, remove the separate flushes and do it only
once, before the backingstore compositor indicates that it is about to
access the textures. This should improve performance a bit, esp. when
doing multisampling since we flush only once then.
A helper function is added to the internal QOpenGLExtensions because
it is highly likely that QQuickWidget will need the same.
Task-number: QTBUG-45106
Change-Id: Ifb405c5723f29f2f6c04df8e15fb70280681755e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
cafd54d34b