Bugfix: add default line Cap and Join

Our render hasn't been including nice rounded edges. This adds the
needed translation from Fabric to plain canvas ctx draw functions. We
can also default these properties to ensure they're always reasonable
for use.

mixins/highlighted_stroke.mixin.js

@@ -4,6 +4,8 @@ module.exports = {
   // in percentage of borderWidth
   outlineWidth: 0.2,
   outlineStyle: "#FFF",
+  strokeLineCap: "round",
+  strokeLineJoin: "round",
 
   /**
    * Provide a custom stroke function that draws a fat white line THEN a
@@ -15,6 +17,10 @@ module.exports = {
     function scale(x) {
       return Math.round(x) / myScale;
     }
+
+    ctx.lineCap = this.strokeLineCap;
+    ctx.lineJoin = this.strokeLineJoin;
+
     ctx.lineWidth = scale(this.borderWidth + outline);
     ctx.strokeStyle = this.outlineStyle;
     ctx.stroke();