isTargetTransparent() now samples the cache directly.

Previously, isTargetTransparent would do a paint from the cache
to the contextCache which has more overhead and is slower.
With this commit, fabricjs will test the cache canvas directly
using x/y coordinates calculated with respect to the target object
center. This is more performant since it avoids an extra render
on every call to isTargetTransparent.

src/canvas.class.js

@@ -492,6 +492,17 @@
      * @return {Boolean}
      */
     isTargetTransparent: function (target, x, y) {
+      if (target.shouldCache() && target._cacheCanvas && !target.isCacheDirty()) {
+        var normalizedPointer = this._normalizePointer(target, {x:x, y:y}),
+            targetRelativePointerX = target.cacheTranslationX + (normalizedPointer.x * target.zoomX),
+            targetRelativePointerY = target.cacheTranslationY + (normalizedPointer.y * target.zoomY);
+
+        var isTransparent = fabric.util.isTransparent(
+          target._cacheContext, targetRelativePointerX, targetRelativePointerY, this.targetFindTolerance);
+
+        return isTransparent;
+      }
+
       var ctx = this.contextCache,
           originalColor = target.selectionBackgroundColor, v = this.viewportTransform;