
When a viewport source rect or destination size is set, `stex->dst_width` gives us the the cropped and/or scaled size. At this step, we need the uncropped/unscaled size however. Note: this is only ever relevant if buffer transform and viewport are used together - otherwise the values are the same. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1836>