Merge pull request #17113 from JanStevens/v4-fix-retina-mixin

Fixes the _image mixing by prefixing the parameters with $

scss/mixins/_image.scss

@@ -19,7 +19,7 @@
 // Short retina mixin for setting background-image and -size.
 
 @mixin img-retina($file-1x, $file-2x, $width-1x, $height-1x) {
-  background-image: url("#{file-1x}");
+  background-image: url($file-1x);
 
   @media
   only screen and (-webkit-min-device-pixel-ratio: 2),
@@ -27,7 +27,7 @@
   only screen and (        min-device-pixel-ratio: 2),
   only screen and (                min-resolution: 192dpi),
   only screen and (                min-resolution: 2dppx) {
-    background-image: url("#{file-2x}");
+    background-image: url($file-2x);
     background-size: $width-1x $height-1x;
   }
 }