padOsd: Figure out label/edition UI positions from unchanged fonts

We can figure out the position to render the text without resorting to
font sizes that make the whole text element sub-pixel size. As the
original labels in the SVG are themed invisible anyway, calculate the
button/ring/strip label and configuration UI position from their unchanged
size/location.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5415
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2288>

data/theme/pad-osd.css

@@ -19,7 +19,6 @@
 .Label {
     stroke: none !important;
     stroke-width: .1 !important;
-    font-size: .1 !important;
     fill: transparent !important;
 }