Gif conversion causes artifacting when gif has alpha transparency

[landaire]

Checklist

• I am reporting an issue in existing functionality that does not work as intended
• I've searched for existing GitHub issues

Description

Conversion from GIF to MP4 causes animated gifs with alpha transparency to have visible artifacting which I believe is caused by not zeroing this buffer for each frame:

Telegram-iOS/submodules/LegacyComponents/Sources/TGGifConverter.m

Line 156 in 1c3b749

CVPixelBufferRef pxBuffer = [self newBufferFrom:imgRef size:renderSize withPixelBufferPool:adaptor.pixelBufferPool andAttributes:adaptor.sourcePixelBufferAttributes];

.

Expected Behavior

The gif should not have artifacts.

Actual Behavior

The gif has artifacts.

IMG_0562.MP4

Steps to Reproduce

1. Save this file somewhere: https://gist.githubusercontent.com/landaire/4a1ea295c353ef85277c12e35d95fe53/raw/8643a704de1f0bf8532847877001033c25ebcf8d/gistfile1.txt
2. Decompress it using base64 -d gistfile1.txt | xz -d > alienpls.gif
3. Send it using Telegram for Mac or iPhone
4. Observe the artifacts.