plugins/screencast: Make grabTexture() more nicer to memfd code path

OutputTransform::Normal is handled by doGrabTexture().

If the texture transform is neither normal nor flip-y, the GPU is going
to be used to transform the texture, however since it doesn't flip the y
axis, doGrabTexture() will flip the y axis on the cpu side. To fix that,
make the contents of backingTexture mirrored vertically.


(cherry picked from commit 7297e622)