Add encoding for animated PNGs

[RunDevelopment]

Closes #1474.

This PR adds a simple APNG encoder and an example showing how to encode animations using PNG and GIF (see below).

The "encoder" is really just two new functions on PngEncoder: encode_frames and try_encode_frames. The names and API were inspired by GifEncoder.

The code is relatively simple. I just wired up what the png crate already supports. The code converting our delays to PNG delays is a little more complex though. The basic idea is that If the delay fraction cannot be represented exactly, then saturate the numerator or denominator to u16::MAX. This works but doesn't get the best possible approximation in general. IMO, this function should be a method on Delay (e.g. as to_seconds_ratio_u16), but we can talk about that later.

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The animation created by the new example looks like this btw:

PNG (58 KB) vs GIF (12 KB) with 50 frames of animation

Hidden by default ^{because I find moving things annoying while reading}

Gif looks worse because 1-bit alpha. This is also the reason why the PNG is larger. Most of the information of the animation is in the alpha channel.