Custom backgrounds for advancement screens don't get copied over

[Zekiu]

Mods that add custom advancements with custom backgrounds don't have their background copied to the resource pack. Manually extracting the assets from the mod's jar and putting them in the resource pack fixes the broken backgrounds.

Here's the same screen with the extracted assets:

Mods used:
MoreAchievements 1.0.0
PolyMC 4.1.2
Polymer 0.2.0-beta.40