Removing glvideomixer-like nuance (it was initially referenced)
and rewriting element since it's not an optimal design at all
from performance point of view.
* Remove wrapper bin (and internal conversion/upload/download elements)
which will waste CPU/GPU resources. Conversion/blending can be done by the
d3d11compositor element at once.
* Add support YUV blending without RGB conversion.
The RGB <-> YUV conversion is completely unnecessary since YUV textures
support blending as well.
* Remove complicated blending operation properties since it's hard
to use from application point of view. Instead, adding "operator" property
like what compositor element does.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2631>
