Group Layer Tree Matching and Linked Cel Texture Extraction

[exokem]

Description

This pull request includes two relatively minor changes. One for the assignment of layers as children to group layers, and another for the extraction of textures from linked cels.

1. Group Layer Child Assignment

Previously, layers would always be added as children to the last-read group layer, regardless of their level relative to that group. As a result, layer structures like this:

A
. B
. C
... D

could end up looking more like this at runtime:

A
. C
... D
..... B

This PR introduces a change that caches one last-read group layer per level. With this change, layers will be added to the last-read group layer whose level is one less than theirs, if such a group layer has previously been read. This should at least ensure that the actual hierarchy is preserved in more cases.

2. Linked Cel Texture Extraction

This PR also adds a simple check for linked cels in the ExtractCel extension for the AsepriteCel class.
Instead of throwing an exception for instances of the AsepriteLinkedCel class, the extension will now extract the texture from the cel that a linked cel is linked to. While it is true that linked cels do not contain pixel data, the extension is more useful if automatically resolves the actual cel reference.

[AristurtleDev]

Thanks @exokem, I agree, the extraction from the LinkedCel is a better experience for the consumer.