ui/gtk: Document scale and coordinate handling

ui/gtk: Document scale and coordinate handling

The existence of multiple scaling factors forces us to deal with various
coordinate systems and this would be confusing. It would be beneficial
to define the concepts clearly and use consistent representation for
variables in different coordinates.

Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
Message-ID: <20250511073337.876650-2-weifeng.liu.z@gmail.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
(cherry picked from commit 9498e2f7e1a247557cfa0f830a86c398a23c6809)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>