QMesh: source is now a QUrl

Change-Id: I14d50550971836be2c5d8942c5240a6b12f8bc8b
Task-number: QTBUG-41527
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
QFrameAllocatorQueue fix

We need to have cached frames + 1 QFrameAllocators in a QFrameAllocatorQueue
otherwise we may be clearing the QFrameAllocator of the frame we are currently
trying to render which isn't a good thing. This issue was more pronounced with
Qt compiled in release mode.

Change-Id: I1eb50cfa79bbdbdf3df0c005069b4b1f8ae99ddf
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>