Use the same region for equal images in Texture Atlas importer

[guilhermefelipecgs]

Godot version:
1120b0b

OS/device including version:
5.0.7-arch1-1-ARCH

Issue description:
Just a suggestion, atlas pack could be even better if we don't repeat the same textures like this example below:

I work with thousands of frames and some of them are repeated for some animations.

[harrisyu]

Yes, I think that's the last step that we don't need external texture packer anymore.