awful.layout.inc: Hide API breakage

In commit 3cbdc2a79f, the argument order for awful.layout.inc was changed
from (layouts, i, s) to (i, s, layouts), so that layouts can become an optional
parameter. However, this change (obviously) breaks user configs.

To hide this breakage, we assume the old argument order if the number i is a
table. This cannot break anything, since the operator "+" will error out on
tables anyway. :-)

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/layout/init.lua.in

@@ -67,6 +67,11 @@ end
 -- @param s The screen number. 
 -- @param[opt] layouts A table of layouts.
 function layout.inc(i, s, layouts)
+    if type(i) == "table" then
+        -- Older versions of this function had arguments (layouts, i, s), but
+        -- this was changed so that 'layouts' can be an optional parameter
+        layouts, i, s = i, s, layouts
+    end
     local t = tag.selected(s)
     local layouts = layouts or layout.layouts
     if t then