added css3 background-clip mixin

lib/compass/frameworks/compass/stylesheets/compass/css3/_background_clip.sass

@@ -0,0 +1,16 @@
+// Background Clip
+// clip the background (image and color) at the edge of the padding or border
+//  - [padding-box | border-box]
+//  - browser defaults to border-box, mixin defaults to padding-box
+
+//  override constants to change defaults
+!default_background_clip ||= "padding-box"
+
+=background-clip(!clip = !default_background_clip)
+  // webkit and mozilla use the deprecated short [border | padding | content]
+  !deprecated= "padding"
+  @if !clip == "border-box"
+    !deprecated= "border"
+  background-clip= !clip
+  -webkit-background-clip= !deprecated
+  -moz-background-clip= !deprecated