[Feature Request] Transparency for BitmapBackends

[wischi-chr]

What is the feature ?

Create images/plots with transparent backgrounds. The BitmapBackend should come/support a PixelFormat with alpha channel.

(Optional) Why this feature is useful and how people would use the feature ?

I often create plots for presentations and websites and I'd like the natural background of the page (doesn't have to be white, could be beige) to show through the plot.

Explain why this feature is important
PNG supports alpha channels but currently there is (unless I missed it) no way of generating PNGs with a transparent background.

(Optional) Additional Information