Add multicolour, or monochrome/modulate-able, rock assets

[wjt]

We currently have this very nice rock:

(and a version that's in the water that we don't currently use)

In res://scenes/quests/lore_quests/quest_001/1_music_puzzle/music_puzzle.tscn, where this rock is used, we recolour 6 instances of it with the modulate property:

The problem is, because the original rock is quite dark, the rocks all come out in quite muted colours. For example, the second rock has this modulate colour:

The rock uses four shades of turquoise, plus a slightly-blue near-black grey for the border.

I think these colourful rocks would look better if either:

1. We actually had a collection of rocks in different colours, hand-picked, and stopped using modulate to make them visually distinct
2. We recoloured the asset to be monochrome, with the brightest colour being set to white and the other three being appropriate shades of grey, so that they can be recoloured arbitrarily with modulate.

I am no artist so I am the wrong person to say which approach would be better!

XP Summary (total 225)

Subdomain	Skill	XP
Art: Aesthetics & Visual Design	Problem Solving	75
Art: Aesthetics & Visual Design	Creativity & Design Thinking	75
Art: Aesthetics & Visual Design	Technical Literacy	75

[manuq]

@wjt option B: use the new CelShadingRecolor node!

[wjt]

Oh yes! I think it would need to support four colours though since the rock has 4 colours.

[wjt]

It kind of works!

[manuq]

Oh right after inspecting the PNG I see that this rock has 4 shades

[wjt]

I opened the rock in GIMP and changed its colour mode to greyscale. The result looks better. So I'll submit a PR for that, and someone more artistically gifted than me can always do something fancier later.