move renderLayerTo to Renderable

Move renderLayerTo to Renderable.

lib/extract/extractStroke.js

@@ -20,10 +20,6 @@ export default function(props) {
 
     let strokeWidth = +props.strokeWidth;
 
-    if (_.isNil(props.strokeWidth)) {
-        strokeWidth = null;
-    }
-
     let strokeDasharray = props.strokeDasharray;
 
     if (typeof strokeDasharray === 'string') {
@@ -41,7 +37,7 @@ export default function(props) {
         strokeLinecap: caps[props.strokeLinecap] || 0,
         strokeLinejoin: joins[props.strokeLinejoin] || 0,
         strokeDasharray: strokeDasharray || null,
-        strokeWidth: strokeWidth,
+        strokeWidth: strokeWidth || null,
         strokeDashoffset: strokeDasharray ? (+props.strokeDashoffset || 0) : null,
         strokeMiterlimit: props.strokeMiterlimit || 4
     };