Remove a sprite from its context

Property definitions

gamnit $ SpriteSet :: unmap_sprite
	# Remove a sprite from its context
	private fun unmap_sprite(sprite: Sprite)
	do
		var context = sprite.context
		assert context != null
		context.sprites.remove sprite

		sprite.context = null
		sprite.sprite_set = null
	end
lib/gamnit/flat/flat_core.nit:962,2--971,4