Record: Make sure the frames we encode are in an expected format

QImage does not support BGRA32, which is what we are feeding from the
dmabuf. Have OpenGL download a GL_RGBA and put it into a QImage without
trying to guess what's on the other side, which just works.

While at it, warn when a format conversion defaults, so we can know
which values we need to have under control.


(cherry picked from commit b8ec58e8)