Add isometric cell shape mode to AStarGrid2D.

[Chaosus]

I know some users are requesting this. This will ease up to set up this class to work with isometric TileMap's.

So I've added a new property cell_shape (just like the property in the TileSet) and enum for this CellShape which has 3 values: CELL_SHAPE_SQUARE (default) and CELL_SHAPE_ISOMETRIC_RIGHT/CELL_SHAPE_ISOMETRIC_DOWN (corresponds to TILE_LAYOUT_DIAMOND_RIGHT and TILE_LAYOUT_DIAMOND_DOWN enum of the TileSet respectively).

This will simply modify the resulted world coordinates and do not affect the id layout of the grid (the update method call still required if changed).

You can check the small demo project which demonstrates this change:
IsometricAStarGrid2D.zip

[smix8]

Not that involved with AStar2D or Grid uses but from what I could test works fine.

[akien-mga]

Thanks!