Deleted post
Basically, for the level:
1) Render the paint (as particles) to a texture (a viewport in Godot) called PaintTexture.
2) Render the PaintTexture to a final texture that doesn't clear after each frame (clear mode = "next frame" in Godot).
3) Render only the background of the level in black to clear it.
And for the spinners:
1) Cut out (using UV coords) the current part of PaintTexture that corresponds to the location of the spinner.
2) Multiply/mask it by the spinner sprite, and render this to a final texture (one per spinner) that doesn't clear after each frame.
3) Rotate the final texture to make it spin.