st/theme-node-drawing: Don't skip square top left inset shadow corners

When the inner radius of the top left corner is 0, the corresponding arc
does not get drawn, resulting in a path going from the bottom left
corner to the top right corner when the path is closed.

Fix this by always including a line_to from the bottom left to the top
left corner.

Fixes: dde13b30c ("st/theme-node-drawing: Trace the correct path for inset box shadows")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7996

src/st/st-theme-node-drawing.c

@@ -972,6 +972,11 @@ paint_inset_box_shadow_to_cairo_context (StThemeNode     *node,
       int inner_radius;
 
       inner_radius = MAX (0, outer_radius[ST_CORNER_TOPLEFT] - shadow_spec->spread);
+
+      cairo_line_to (temp_cr,
+                     shrunk_extents_x1,
+                     shrunk_extents_y1 + inner_radius);
+
       if (inner_radius > 0)
         {
           cairo_arc (temp_cr,