Add shadow tiles to scenes

[wjt]

In #1780 I added a TileSet for shadows at the base of cliffs.

We can now gradually update scenes in the game to use this. So far I have only added shadows below cliffs in Fray's End. This scene can be used as an example.

Not all scenes have to be done at once: we can make subtasks for each scene or group of scenes. The XP scores below should be propagated to subtasks.

For each scene that has cliffs on top of land but no shadows:

1. Add a new TileMapLayer called Shadows, directly above the cliff layer in the scene tree (so that in the scene it is rendered underneath the cliff. (If there are multiple numbered levels of cliffs, add multiple shadow layers.)

   

2. In the inspector, set the Tile Set to tiles/shadows.tres

   

3. Paint shadows where the cliffs meet the ground:

   

   Deselect the TileMapLayer node to see how it looks:

   

XP Summary (total 200)

Subdomain	Skill	XP
Game Design: Game Feel & UX	Problem Solving	50
Game Design: Game Feel & UX	Creativity & Design Thinking	75
Game Design: Game Feel & UX	Technical Literacy	75