Copy opacity control onto buttons 6 and 7, not requiring Alt to be pressed.
(These are usually the side buttons, but may be the horizontal wheel).

Signed-off-by: Darren Salt <linux@youmustbejokin.demon.co.uk>

Index: src/events.c
===================================================================
--- src/events.c	(revision 22201)
+++ src/events.c	(working copy)
@@ -722,6 +722,16 @@
             clientUnshade (c);
         }
     }
+    else if (ev->button == 6)
+    {
+        /* Mouse wheel scroll left, or left side button */
+        clientDecOpacity(c);
+    }
+    else if (ev->button == 7)
+    {
+        /* Mouse wheel scroll right, or right side button */
+        clientIncOpacity(c);
+    }
 }
 
 static void