Fix breakage of tst_qquickwindow::grab(invisible)

Instead of waiting for -drawRect to call -setView on the
NSOpenGLContext, we go ahead and attempt to set the context as soon as
possible.  If it is indeed required that we call -drawRect first then
will try to call -setView again during -drawRect with the new
NSOpenGLContext.

Change-Id: I33d9f2ba241b49e8cfa6c9156dd5bf5e4cc6b164
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>