Update to allow color for transparent png images

[JewrassicPark]

Hey,

I needed this for a project so I added it to my hacked up phppres but it's a small change so thought you might want to merge it.

In the writeShapeDrawing function of Writer/PowerPoint2007/PptSlides.php

You can add

$this->writeFill($objWriter, $shape->getFill());

just before writeBorder

Same way as in writeShapeText (though that now appears to be in the abstract file).

[Progi1984]

@JewrassicPark Fixed in develop branch (Reader / Writer - ODPresentation & PowerPoint2007)