Animated APNG stickers with palette are not animated

• I have searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

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Bug description

Some animated APNG stickers are not animated on Signal for iOS. While digging, I realized that the common thread between faulty stickers is that they contain a palette (i.e. PNG with a tRNS chunck).

Having a palette is very important for animated stickers, as it allows to save a lot of weight, thus allowing stickers with higher resolution/more frames to be uploaded.

Note that APNG stickers with palette do work on Signal on all other platforms.

Here are some examples:

Demo sticker with palette

This sticker is not animated on iOS. ❌
https://signal.art/addstickers/#pack_id=b020b5f29795e931cf11c4450d12ebca&pack_key=f2a19052ed4077ffbe59d7374a6a7ed7e433854abb5b5844f0af5eaf3c7f5ade

% pngcheck sticker_with_palette.png 
OK: sticker_with_palette.png (304x304, 8-bit palette+trns, non-interlaced, 24.4%).
% du -h sticker_with_palette.png 
72K	sticker_with_palette.png

Demo sticker without palette (RGB)

This sticker is animated on iOS. ✔️
https://signal.art/addstickers/#pack_id=f30476e7bab76d9b73c289c97bc6fdb4&pack_key=0bb6189c1c7ab04b56a3aa59a69de5101a4a6b0dec54cc3c880732b7be7546d7

% pngcheck sticker_rgb.png
OK: sticker_rgb.png (300x300, 32-bit RGB+alpha, non-interlaced, 67.4%).
% du -h sticker_rgb.png
116K	sticker_rgb.png

Example pack containing both with and without palette stickers

This pack's only sticker that is animated on iOS is the one without a palette:

https://signal.art/addstickers/#pack_id=c8b7a540d40b85697a6f80491f78a04a&pack_key=75aa857b1bd3d7553320da129f4db97a0c10b6f4fceb9264b22d66f4292b318e

% pngcheck *.png
OK: 0.png  (270x300, 8-bit palette+trns, non-interlaced, 91.5%).
OK: 1.png  (320x300, 8-bit palette+trns, non-interlaced, 90.3%).
OK: 2.png  (334x300, 8-bit palette+trns, non-interlaced, 81.7%).
OK: 3.png  (370x300, 8-bit palette+trns, non-interlaced, -13.6%).
OK: 4.png  (270x300, 32-bit RGB+alpha, non-interlaced, 91.7%).
OK: 5.png  (270x300, 8-bit palette+trns, non-interlaced, 82.1%).
OK: 6.png  (270x300, 8-bit palette+trns, non-interlaced, 84.0%).
OK: 7.png  (300x300, 8-bit palette+trns, non-interlaced, 90.7%).
OK: 8.png  (320x300, 8-bit palette+trns, non-interlaced, -6.7%).
OK: 9.png  (270x300, 8-bit palette+trns, non-interlaced, 91.3%).
OK: 10.png (270x300, 8-bit palette+trns, non-interlaced, 89.9%).
OK: 11.png (280x300, 8-bit palette+trns, non-interlaced, 63.4%).
OK: 12.png (270x300, 8-bit palette+trns, non-interlaced, 76.4%).
OK: 13.png (280x300, 8-bit palette+trns, non-interlaced, 59.6%).
OK: 14.png (270x300, 8-bit palette+trns, non-interlaced, 71.4%).
OK: 15.png (270x300, 8-bit palette+trns, non-interlaced, 89.3%).
OK: 16.png (300x300, 8-bit palette+trns, non-interlaced, 68.3%).
OK: 17.png (270x300, 8-bit palette+trns, non-interlaced, 73.5%).
OK: 18.png (280x300, 8-bit palette+trns, non-interlaced, 59.7%).
OK: 19.png (290x300, 8-bit palette+trns, non-interlaced, 68.0%).
OK: 20.png (270x300, 8-bit palette+trns, non-interlaced, 42.6%).
OK: 21.png (300x300, 8-bit palette+trns, non-interlaced, 69.8%).
OK: 22.png (270x300, 8-bit palette+trns, non-interlaced, 50.6%).
OK: 23.png (270x300, 8-bit palette+trns, non-interlaced, 86.7%).

No errors were detected in 24 of the 24 files tested.

Steps to reproduce

• Download a pack containing stickers with a palette on a iOS device (see example above)

Actual result: animated stickers are not animated

Expected result: animated stickers are animated

Device info

Device: iPhone 11 Pro

iOS version: 14.1

Signal version: 3.21.0.1

Note that I don't own an iOS device, so I can't provide debug log or further information.

No source files, tests, or entry points are named. Start by reproducing the issue on iOS 14.1 with the linked palette and RGB sticker packs, then compare their animation behavior. Done means APNG stickers containing a palette and tRNS chunk animate on iOS as the RGB examples do.