Improve support for a number of TMX/Tiled features

[r1chardj0n3s]

We made use of an isometric map in our PyWeek entry https://github.com/r1chardj0n3s/masa-01 which required some improvements both to pytiled-parser (PR pythonarcade/pytiled_parser#24 ) and to arcade itself both for some issues around positioning of tiles in isometric layout, but also to add support for image layers.

[pvcraven]

After merge, seems like unit tests are failing:

ImportError while loading conftest 'C:\Users\craven\Desktop\WebServer\arcade\tests\test_gui\conftest.py'.
test_gui\__init__.py:8: in <module>
    import arcade
..\arcade\__init__.py:242: in <module>
    from .tilemap import get_tilemap_layer
..\arcade\tilemap.py:169: in <module>
    def _get_image_source(tile: Union[pytiled_parser.objects.Tile, pytiled_parser.objects.ImageLayer],
E   AttributeError: module 'pytiled_parser.objects' has no attribute 'ImageLayer'

[einarf]

It depends on : https://github.com/Beefy-Swain/pytiled_parser/pull/24/files

[einarf]

@r1chardj0n3s I think this is the current dev branch in pytiled_parser : https://github.com/Beefy-Swain/pytiled_parser/tree/rf/caster

The plan was to integrate with this version as it is feature complete.

[pvcraven]

This PR needs re-opened somehow, not and only merged when pytiled parser stuff is also merged.

[einarf]

@Cleptomania mentioned they could do a release of pytiled_parser with the change from @r1chardj0n3s

[r1chardj0n3s]

Agh, my deepest apologies for omitting that important dependency information in the PR note!