Some support for APNG files is required

[drj11]

(this is a revival of #63 since this is now a personal itch)

On macOS if you use Preview to convert a PDF to a PNG file; it gives you a multi-image PNG file. I think this is an APNG file.

Think about what basic support for this might look like and add it to the library.

[drj11]

pipcat can be used to extract just the first image (but it also removes things like the colour profile which maybe you wanted):

python3 code/pipcat /Users/drj/Desktop/FY-2018-FY-2019-Proposed-CIP-Budget-Book.png > first-image.png

[drj11]

APNG spec https://wiki.mozilla.org/APNG_Specification