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.