Improve APNG support

[andrews05]

Two improvements to APNG support:

1. fcTL and fdAT chunks are parsed into Frames, allowing us to consolidate multiple fdATs for a frame and re-sequence the chunks. This means we can now recompress all frames, rather than only in the case where there's a single fdAT for the frame.
2. Following on from this, we can now also re-filter the frames, including alpha optimisation when enabled. We use the same filter that was selected for the IDAT.

Reductions are still disabled.

Results on a set of 32 apngs, optimised with -sao6:

source	size
original	20,022,193
master	19,412,724 (3.0% smaller)
with full recompression only	16,494,618 (17.6% smaller)
with refiltering	15,583,118 (22.2% smaller)
apngopt* (default)	13,520,262 (32.5% smaller)
apngopt + oxipng	13,341,852 (33.4% smaller)

(There was a particularly large and poorly compressed file in the set which may be skewing the results a little.)

*Note that apngopt does not guard against writing a larger file than the input. The results here could be slightly smaller if I retained the originals where they were better.

[edit] A second set:

source	size
original	165,721,029
master	165,048,775 (0.4% smaller)
with full recompression only	163,422,620 (1.4% smaller)
with refiltering	162,850,831 (1.7% smaller)
apngopt (default)	164,919,387 (0.5% smaller)
apngopt + oxipng	162,474,452 (2.0% smaller)

[ace-dent]

@andrews05 - exciting to see some effort around APNG. Thanks! 🙏

[AlexTMjugador]

As I initially thought, this looks great overall! I have just a couple of comments to share 🙂