Handle Hexagon-based tilemaps

[jamesnunn]

I've tried using Gaea with my hex map but encountering some strange behaviour when using the chunking tools. It seems the actor position is calculated with square tiles in mind and there are multiple other places in the code where hexes aren't handled (e.g. grid_2d.gd SURROUNDING).

I'd be willing to contribute but this is my first time with Godot (but an experienced developer otherwise) so if you have pointers to parts of the code I'd need to focus on that will help me grok from there onwards I will give it a shot.