GH 1151: export GIF

[ejulio-ts]

Fixes #1151

This allows diagrams to be exported as animated GIFs.

There is a minor refactor to introduce exportExtension in order to avoid having many file extensions around the code. It is the first commit and then further commits have minor changes to the initial idea.

The overall export piece needs some refactoring, as the PNG code is duplicated in all formats (pptx, png, pdf, gif). Similarly, the sequence in which we export multiple boards. As it seemed a bigger change, I left it out for now and may revisit it later.

The basic idea is:

• Export all boards as PNGs
• Convert PNGs to Bitmaps (GIF frames) with 255 colors (this is the reason I had to add a new library)
• Make all GIF frames the same size (largest width and height of all boards)
• Center the diagrams in the frame and keep a white background
• Export the animation with an infinite loop

Note that this can "slow" as there are many image manipulations going on and it depends on the image size. I considered using goroutines here, but left out for simplicity at first. The examples below took between 5s-8s to compile.

8s

5s

[alixander]

amazing that it works, hell yeah.

Can you add a periodic message that it's still running?
https://github.com/ejulio-ts/d2/blob/d1bb3cc575a9f6f1bc5d00067ff8e39a61a2e5de/lib/png/png.go#L93