videomixer: get height via GST_VIDEO_FRAME_HEIGHT, not _WIDTH
videomixer: get height via GST_VIDEO_FRAME_HEIGHT, not _WIDTH

https://bugzilla.gnome.org/show_bug.cgi?id=687330

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good