Add transparency support in images

Working/thinking direction:

1. Extract the alpha channel from a png or gif image
2. Create a new greyscale image based on the alpha channel
3. Save the image as a jpg image (lossing the alpha in the process)
4. Set the SMask (softmask) in the PDF image dictionary that points towards the new greyscale image. To relink the alpha channel to the image.
5. **Tada** Transparent image