stylecontext: Add style property fallback for gtk-cursor-aspect-ratio

The gtk-cursor-aspect-ratio GtkSetting was only recently introduced and
replaced the cursor-aspect-ratio style property. This was causing old
configurations that used CSS to modify this property to no longer apply.
This commit introduces a fallback from the GtkSetting to the style
property if the value of the setting is unchanged.

Related: #3441

gtk/gtkstylecontext.c

@@ -2916,6 +2916,14 @@ gtk_render_insertion_cursor (GtkStyleContext *context,
                 "gtk-cursor-aspect-ratio", &aspect_ratio,
                 NULL);
 
+  /* Fall back to style property if the GtkSetting property is unchanged */
+  if (aspect_ratio == 0.04f)
+    {
+      gtk_style_context_get_style (context,
+                                   "cursor-aspect-ratio", &aspect_ratio,
+                                   NULL);
+    }
+
   keymap_direction = gdk_keymap_get_direction (gdk_keymap_get_for_display (gdk_screen_get_display (priv->screen)));
 
   pango_layout_get_cursor_pos (layout, index, &strong_pos, &weak_pos);
@@ -3003,6 +3011,14 @@ gtk_draw_insertion_cursor (GtkWidget          *widget,
                 "gtk-cursor-aspect-ratio", &aspect_ratio,
                 NULL);
 
+  /* Fall back to style property if the GtkSetting property is unchanged */
+  if (aspect_ratio == 0.04f)
+    {
+      gtk_style_context_get_style (context,
+                                   "cursor-aspect-ratio", &aspect_ratio,
+                                   NULL);
+    }
+
   draw_insertion_cursor (context, cr,
                          location->x, location->y, location->height,
                          aspect_ratio,