Fix glvideomixer black output

commit cdd5470f

glvideomixer: fix black output after display changes

Partial revert of 55e80b550e4ad02aaf44b30ec8b03da9ee485ac6

https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/743

commit ebf8198a

glvideomixer: intersect with template caps when updating

Downstream may provide extra things when we ask it for caps that we may
need to remove.

Fixes the following pipeline warning:

gltestsrc ! glvideomixerelement ! glimagesinkelement

** (gst-launch-1.0:908581): WARNING **: 13:53:28.518: glvideomixer0: update_src_caps returned caps which are not a real subset of the filter caps

Fixes #743 (closed)