screencasting: on memfd, skip the QImage step

We were using QImage as an intermediary step. GL -> QImage -> spa
buffer. While it abstracted things out neatly and eventually helped with
debugging, it was unnecessary and woudl present some handicaps, such as
the lack of a QImage::Format_BGRA.
So we just it out to download right into the buffer.

BUG: 466655
(cherry picked from commit 12145458)