winrt: Fix video playback

Rendering should only be activated once the video frame size is known.
Otherwise the texture is created with an invalid size and rendering will
be stopped. This happens for some codecs where
MF_MEDIA_ENGINE_EVENT_LOADEDMETADATA is not known instantly after
loading video files.

Task-number: QTBUG-57707
Change-Id: Ic2347c8606239770e129544fd228b96929ed6198
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>