shape: Add circle radius parameter.

lib/gears/shape.lua

@@ -328,9 +328,10 @@ end
 -- @param cr A cairo context
 -- @tparam number width The shape width
 -- @tparam number height The shape height
-function module.circle(cr, width, height)
-    local size = math.min(width, height) / 2
-    cr:arc(width / 2, height / 2, size, 0, 2*math.pi)
+-- @tparam[opt=math.min(width  height) / 2)] number radius The radius
+function module.circle(cr, width, height, radius)
+    radius = radius or math.min(width, height) / 2
+    cr:arc(width / 2, height / 2, radius, 0, 2*math.pi)
     cr:close_path()
 end