Add another head shape to character randomizer

[wjt]

Threadbare has a character randomizer to create distinct NPCs from a small number of component parts. See it in action in game_elements/characters/components/character_randomizer_test.tscn.

Currently the character randomizer has five head shapes:

We would like more!

The way these are implemented is that there is a sprite sheet for each head in res://scenes/game_elements/characters/npcs/components/, and then a corresponding SpriteFrames in res://scenes/game_elements/characters/components/sprite_frames/, which are added to the RandomTextureSpriteBehavior in res://scenes/game_elements/characters/npcs/townie.tscn. The sprite sheets use three colour shades, plus black. Then, in the game, a chroma-key shader replaces those three shades of yellow with one of a number of alternative palettes. The sprite sheets are exported from a single Aseprite file at scenes/game_elements/characters/npcs/components/townie.aseprite.

So to add a new head shape:

1. Open scenes/game_elements/characters/npcs/components/townie.aseprite in Aseprite, Pixelorama or compatible software
2. Add a new head layer
3. Draw a new head shape using the same four colours
4. Export the spritesheet
5. Add a SpriteFrames
6. Update townie.tscn to include it in the randomized choices

@manuq created the existing assets & the game system that uses them, so he is the best person to ask for pointers!

XP Summary (total 450)

Subdomain	Skill	XP
Art: Animation & Effects	Problem Solving	150
Art: Animation & Effects	Creativity & Design Thinking	150
Art: Animation & Effects	Technical Literacy	150