Fix issue with using different tilesets that use the same tile IDs

[lmReef]

Just made a quick fix for an issue I was having where if you try to use different tilesets in Tiled that contain tiles with identical IDs the latter tile gets skipped.

Note I made the changes assuming that replacing the manually tracked _atlas_source_counter can be replaced with the return value of TileSet.add_source() where it gets the next source id itself. I'm not too sure if this is something ya want to add but it fixed an issue for me so thought I'd put up a pr :) Feel free to change it if you like

Example

In Tiled Editor

Godot scene result before changes

Godot scene result after changes

Related Godot console errors

https://github.com/godotengine/godot/blob/2f73a059cefadcd944b6874f2557ec82e46a562d/scene/resources/tile_set.cpp#L475

[Kiamo2]

Looks promising, I'll test that with all my examples (which can take a while).

[Kiamo2]

Very smart fix, no more atlas source counting!