Add clip-shadow-above configuration and wintype option

Added the new `clip-shadow-above` configuration and wintype option.
These allow the user to select windows to clip from the shadow region of
other windows, i.e. don't paint shadows on top of them.

This should provide a more useful and userfriendly alternative to the
deprecated `shadow-exclude-reg` option — especially for docks and bars.

picom.sample.conf

@@ -51,6 +51,9 @@ shadow-exclude = [
   "_GTK_FRAME_EXTENTS@:c"
 ];
 
+# Specify a list of conditions of windows that should have no shadow painted over, such as a dock window.
+# clip-shadow-above = []
+
 # Specify a X geometry that describes the region in which shadow should not
 # be painted in, such as a dock window region. Use
 #    shadow-exclude-reg = "x10+0+0"
@@ -396,6 +399,10 @@ log-level = "warn";
 #     normally won't be able to see. Useful when the window has parts of it
 #     transparent, and you want shadows in those areas.
 #
+#   clip-shadow-above:::
+#     Controls wether shadows that would have been drawn above the window should
+#     be clipped. Useful for dock windows that should have no shadow painted on top.
+#
 #   redir-ignore:::
 #     Controls whether this type of windows should cause screen to become
 #     redirected again after been unredirected. If you have unredir-if-possible
@@ -405,7 +412,7 @@ log-level = "warn";
 wintypes:
 {
   tooltip = { fade = true; shadow = true; opacity = 0.75; focus = true; full-shadow = false; };
-  dock = { shadow = false; }
+  dock = { shadow = false; clip-shadow-above = true; }
   dnd = { shadow = false; }
   popup_menu = { opacity = 0.8; }
   dropdown_menu = { opacity = 0.8; }