shaped-texture: Viewport update calculation fixes

If only a viewport destination size is set, the noop viewport has
to take the buffer scale into account.

If a viewport source but no viewport destination size is set, the
destination size is that of the viewport source, not of the whole
texture.


(cherry picked from commit 52547cb9)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1808>