Add support for text kerning

[olivervogel]

Letter spacing or text kerning should at least be possible with Imagick:

https://www.php.net/manual/en/imagickdraw.settextkerning.php

However, this is not possible with the GD library and would have to be implemented manually. Unfortunately, I currently doubt that I can achieve visually appealing and typographically correct results with GD driver myself.