Use transparent background instead of white (for better look in the browser)

git-svn-id: http://root.cern.ch/svn/root/trunk@40139 27541ba8-7e3a-0410-8455-c3a389f83636

icons/branch_t.xpm

@@ -5,7 +5,7 @@ static char*branch_t[]={
 "# c #008000",
 "b c #808000",
 "a c #008080",
-". c #ffffff",
+". c None",
 "...........#....",
 ".........a#a....",
 "........a#aa...b",