Fix SVG size calculation

It was being incorrectly calculated based on devicePixelRatio.

Fixes LLK/scratch-gui#73. Thanks to @cwillisf for figuring it out!

src/SVGSkin.js

@@ -34,10 +34,10 @@ class SVGSkin extends Skin {
     }
 
     /**
-     * @return {[number,number]} the "native" size, in texels, of this skin.
+     * @return {[number,number]} the natural size, in Scratch units, of this skin.
      */
     get size () {
-        return [this._svgRenderer.canvas.width, this._svgRenderer.canvas.height];
+        return this._svgRenderer.size;
     }
 
     /**

src/svg-quirks-mode/svg-renderer.js

@@ -72,6 +72,13 @@ class SvgRenderer {
         this._draw();
     }
 
+    /**
+     * @return {[number,number]} the natural size, in Scratch units, of this SVG.
+     */
+    get size () {
+        return [this._measurements.width, this._measurements.height];
+    }
+
     /**
      * Transforms an SVG's text elements for Scratch 2.0 quirks.
      * These quirks include: