Fix item global matrix error (#1562)

Bug happen when item is drawn after an empty symbol that should be drawn
in a separate canvas context (partial opacity or special blend mode).
As bounds are empty, symbol drawing process is interrupted but its
global matrix is not removed from the stack.
Closes #1561

src/item/Item.js

@@ -4327,8 +4327,12 @@ new function() { // Injection scope for hit-test functions shared with project
             // Apply the parent's global matrix to the calculation of correct
             // bounds.
             var bounds = this.getStrokeBounds(viewMatrix);
-            if (!bounds.width || !bounds.height)
+            if (!bounds.width || !bounds.height) {
+                // Item won't be drawn so its global matrix need to be removed
+                // from the stack (#1561).
+                matrices.pop();
                 return;
+            }
             // Store previous offset and save the main context, so we can
             // draw onto it later.
             prevOffset = param.offset;

test/tests/Item_Bounds.js

@@ -789,3 +789,12 @@ test('group.internalBounds with child and child.applyMatrix = false (#1250)', fu
     equals(group.internalBounds, new Rectangle(0, 0, 250, 250),
             'group.internalBounds after scaling item1');
 });
+
+test('#1561 item._globalMatrix on item after empty symbol', function(){
+    var symbol = new SymbolItem(new Path());
+    symbol.opacity = 0.5;
+    symbol.skew(10);
+    var item = new Path.Circle(new Point(0,0), 10);
+    view.update();
+    equals(item._globalMatrix, new Matrix());
+});