Fixed issues with png transparency.

We noticed that image transparency was being lost when uploading images from LightSpeed, despite the fact that IMAGE_BACKGROUND was null. It turns out that it was being lost in two ways: first, whenever Images::SaveImageData() would save the $blbImage to disk, and second whenever Image_GD_Driver attempted to create a transparent blank image with imagecreatetransparent().

SaveImageData() needs to run imagealphablending() and imagesavealpha() on the $blbImage before saving it.

Image_GD_Driver's original imagecreatetransparent() method did not create a transparent image at all, it was black. We replaced it with a different version.