Uncommented the increment of $imageCounter (#205)

The static variable $imageCounter previously had a constant value of 0 throughout the program as it was never being modified, because it was commented out by mistake.

Now it increments every time a new drawing is being instantiated.

This also fixed a problem when multiple images of same name in the same worksheet are incorrectly displayed.

Fixes #204
Closes #205

src/PhpSpreadsheet/Worksheet/BaseDrawing.php

@@ -139,7 +139,8 @@ class BaseDrawing implements IComparable
         $this->rotation = 0;
         $this->shadow = new Drawing\Shadow();
 
-        // Set image index++self::$imageCounter;
+        // Set image index
+        ++self::$imageCounter;
         $this->imageIndex = self::$imageCounter;
     }