Collision Shape Edges don't align to next image

[TokisanGames]

The edges of our collision meshes don't line up across regions.

We have 1024^2 sized heightmaps. Our HeightmapShape3D collision shape has a size of 1025^2 so that the last row (purple below) bridges the gap to the next collision shape.

However, there's nothing in the code to look at the adjacent map. Frankly I'm surprised that the last row is anything but zero. It seems to have some values but I don't understand from where unless our array indexing is messed up. The fix for this issue is to properly set the last row values to the same as the first row on the next image.

https://github.com/outobugi/Terrain3D/blob/41f5456e4f5eb923738b55c66d275194d7e35408/src/terrain_3d.cpp#L120-L140

Also see #152.