Fix for Transparent PNG files for Symfony sfThumbnailPlugin

One of the project I work on uses Symfony 1.4.  I had a client contact me today saying that when they uploaded transparent png files, the resized files were no longer transparent, instead had a black background.  I thought this was strange and knew it had to be something with resizing of the images using sfThumbnail.  The original images were fine, it was only the resized images that no longer had their transparency. After doing some google searching I came across a number of people with the same issue.  I eventually was led to this...