Update QOpenGLFunctions docs

Mention QOpenGLContext::functions() which is used quite commonly in Qt
itself and also some examples.

Also, make it clear that new instances do not degrade performance since
function resolving is done only once.

Change-Id: I74ac194a0b3956a70e5078dd5a1bcdd277b5cc28
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Avoid calling syncWindow with already destroyed platform windows

Issuing a metacall from swapBuffers is dangerous since the window
may get destroyed by the time the slot is invoked.

This patch changes it to use an event, which is more efficient anyway,
that can be invalidated in case the QXcbWindow is destroy()'ed before
the event is delivered.

Change-Id: I44618ac1cb8b221aecce001ea39702164dcab6a5
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Uli Schlachter <psychon@znc.in>