add a helper function to create TilemapChunkTileData from Y-down tile data

[hxYuki]

Objective

• Add a helper function to import tiles in Y-down format.

Solution

• Add TilemapChunkTileData::from_y_down_tiles
• Add comment on TilemapChunkTileData to explain how the data is interpreted.

[kfc35]

The math looks good although I think i stumbled myself upon the variable names for a little(“upper_start” is actually “earlier” in the data and corresponds to what will be the “higher indices” part of the resulting vec). I don’t have any suggestions in this realm, beyond maybe renaming or maybe adding a comment about the names.

I think to aid with comprehension and to ensure correctness, I would like a couple tests to be written (perhaps two tests with an even chunk_size.y and an odd chunk_size.y).