GStreamer: Remove tags when an invalid QVariant is assigned.

Clear the tag list before syncing with reference set so old values
are removed and explicitly remove invalid QVariants from the reference
set so that the type correction doesn't result in invalid values being
written instead.

Change-Id: I7d1bdc95cd2a2d601720db84c9b3ef629477bc99
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
tst_qcamerainfo: Fix weird symbol in license header

Change-Id: Idff0ffb96fa86dd7faa9ff9b86338c238d4edcb5
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Release videoframe on painter stop()

QVideoSurfaceGLPainter/QVideoSurfaceGlslPainter needs
to release the video frame on stop (just like
QVideoSurfaceGenericPainter already does).

Change-Id: Iaf3eb13eaf51fbc22fab6b1f80db8e8978ac328d
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>