Use first item of an array of tilesets when making a TilemapGPULayer

[ChrisCPI]

This PR

• Fixes a bug

Describe the changes below:

In the createLayer method on a tilemap, it says that a string or tileset (or an array of those) can be passed in for tileset. However, the TilemapGPULayer (according to its documentation) can only support one tileset. And so, when the gpu parameter was true, the GPU layer didn't expect to see an array, and so when passing in an array, it breaks.

Now, when gpu is set to true, it checks if tileset is an array. If it is, then it will only pass in the first item of that array when creating the layer. If the array has more than one item, it gives a warning.

Fix #7158