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 would 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)
Same as !3746 (merged)