gtk3: Do not draw drop shadows on tiled windows

Only draw the 1px border, and transparent shadow for resizing.
This is what upstream does, and was already done for GTK 4 [1].
Do the same for GTK 3 for consistency.

[1] 109c19a

common/gtk-3.0/sass/_common.scss

@@ -3285,6 +3285,13 @@ decoration {
   .tiled-bottom &,
   .tiled-left & {
     border-radius: 0;
+    box-shadow: 0 0 0 1px if($variant == 'dark' or $variant == 'darker', darken($header_bg, 7%), $_wm_border),
+                0 0 0 20px transparent; //transparent control workaround -- GTK issue #3670
+
+    &:backdrop {
+      box-shadow: 0 0 0 1px if($variant == 'dark' or $variant == 'darker', darken($header_bg, 7%), $_wm_border),
+                  0 0 0 20px transparent; // GTK issue #3670
+    }
   }
   .popup & {
     box-shadow: none;