gtk4: Update overshoot drawing mixin

Update old mixin derived from GTK 3, based on current GTK 4 upstream.

Author: jnsh

common/gtk-4.0/4.2/sass/_drawing.scss

@@ -309,32 +309,15 @@
 
   $_big_gradient_length: 60%;
 
-  $_position: center top;
   $_big_gradient_size: 100% $_big_gradient_length;
 
-  @if $p==bottom {
-    $_position: center bottom;
-  }
-
-  @else if $p==right {
-    $_position: right center;
-    $_big_gradient_size: $_big_gradient_length 100%;
-  }
-
-  @else if $p==left {
-    $_position: left center;
-    $_big_gradient_size: $_big_gradient_length 100%;
-  }
-
-  background-image: -gtk-gradient(radial,
-                                $_position, 0,
-                                $_position, 0.6,
-                                from(transparentize($c, 0.8)),
-                                to(transparentize($c, 1)));
+  background-image: radial-gradient(farthest-side at $p,
+                                transparentize($c, 0.8),
+                                transparentize($c, 1));
 
   background-size: $_big_gradient_size;
   background-repeat: no-repeat;
-  background-position: $_position;
+  background-position: $p;
 
   background-color: transparent; // reset some properties to be sure to not inherit them somehow
   border: none;                  //