add mix: null to enable remixing Mapzen icons with different basemaps

Author: nvkelso

themes/bubble-wrap-icons.yaml

@@ -2,6 +2,8 @@ styles:
     mapzen_icon_library:
         base: points
         texture: mapzen_icon_library
+        # To make Refill's colorize-able icons play nice with other styles
+        mix:             # intentionally blank, is parsed as null (no mix)
         blend_order: 1
         draw:
             size: [[13, 14px], [16, 18px], [18, 19px]]