screencastService: Add dmabuf/GL pipeline

screencastService: Add dmabuf/GL pipeline

This pipeline imports dmabufs and does format conversion using GL.
The `video/x-raw(memory:DMABuf)` filter ensures format negotiation
between `pipewiresrc` and Mutter will only succeed if Mutter advertises
dmabuf support as well, falling back to the next pipeline otherwise.

Using this pipeline frees Mutter from downloading buffer content on the
main thread which can have a big impact on compositor performance.
Doing format conversion on the GPU should further improve the overall
performance on most hardware.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2597>