screen-cast-stream-src: Always advertise support for BGRA
Some clients - notably the Gstreamer vapostproc element when using Intel GPUs - only support BGRA, not BGRx. We already assume that we can support this format for window screen casts, and even in case of failure we now have a re-negotiation fallback in place. Thus it's pretty safe to support it for all screen cast types.
The possible duplication in case of window screen casts doesn't seem to be a problem for either Pipewire or existing clients like OBS.
Related:
Edited by Robert Mader