Processing semi-transparent pngs

[williammalo]

When trying to process semi-transparent pngs, a white background is added.
I have a project where I need to upscale images, and waifu2x is the only algorithm that looks good enough, but unfortunately, I can't use it for semi-transparent image :(

[nagadomi]

I fixed this issue and updated the demo-application.

[williammalo]

great! I'm so happy!!! you are doing a great service to mankind

[leilei-]

Thanks. I had to use an annoying imagemagick-based workaround of splitting the alpha to a separate image for Waifu2x to process in a batch, and it wasn't always reliable because Imagemagick is not consistent with reassembling images from components.

[nagadomi]

My implementation is similar to ImageMagick solution.
The alpha channel is scaled with generic image scaler.

[nagadomi]

Fixed #29