Class BaseDrawing imageIndex Should provide modification methods

[luoyangpeng]

This is:

- [ ] a optimization suggestion
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

Class BaseDrawing add function setImageIndex method,Allow users to set image index.
For many identical pictures, set the imageIndex property，export efficiency can be greatly improved.

<?php
 public function setImageIndex($imageIndex)
 {
    $this->imageIndex = $imageIndex;
    return $this;
 }

What is the current behavior?

Currently, the number of pictures is counted as an index, and pictures are reused differently

<?php
 ++self::$imageCounter;
 $this->imageIndex = self::$imageCounter;

[MAKS-dev]

Have look at #1153 (or the older #543).
Still hoping on a official fix for this...