Add isMetricsReady for use in bounds computation

Bounds computations don't need to get a fully updated copy of
the skin's texture. It just needs to know the size and rotation
center, which is what this method can check for.

Default implementation defaults to !!getTexture([100, 100]) for
compatibility with quirks relying on the old behavior. The
getTexture() functions in scratch-render don't fall prey to issues
such as TurboWarp/extensions#2207
so this default implementation is fine.
(TextBubbleSkin almost does, but its drawables are non-interactive)

Closes #16

Author: GarboMuffin

src/RenderWebGL.js

@@ -1636,7 +1636,7 @@ class RenderWebGL extends EventEmitter {
         const drawable = this._allDrawables[drawableID];
 
         /** @todo remove this once URL-based skin setting is removed. */
-        if (!drawable.skin || !drawable.skin.getTexture([100, 100])) return null;
+        if (!drawable.skin || !drawable.skin.isMetricsReady()) return null;
 
         const bounds = drawable.getFastBounds();
 
@@ -1911,7 +1911,7 @@ class RenderWebGL extends EventEmitter {
         if (
             !stampDrawable ||
             !stampDrawable.skin ||
-            !stampDrawable.skin.getTexture([100, 100])
+            !stampDrawable.skin.isMetricsReady()
         ) {
             return;
         }

src/Skin.js

@@ -115,6 +115,17 @@ class Skin {
         return this._emptyImageTexture;
     }
 
+    /**
+     * Determine if the skin's size and rotation center properties are accurate.
+     * Default implementation returns true if getTexture([100, 100]) succeeds
+     * as this indicates that the skin is ready to be rendered. Child classes
+     * should override appropriately if getTexture() is known to be slow.
+     * @returns {boolean} true if size and rotation center are accurate.
+     */
+    isMetricsReady () {
+        return !!this.getTexture([100, 100]);
+    }
+
     /**
      * Get the bounds of the drawable for determining its fenced position.
      * @param {Array<number>} drawable - The Drawable instance this skin is using.