Map texture then static vs dynamic to a SpriteContext

Property definitions

gamnit $ SpriteSet :: contexts_map
	# Map texture then static vs dynamic to a `SpriteContext`
	private var contexts_map = new HashMap4[RootTexture, nullable RootTexture, Bool, Int, Array[SpriteContext]]
lib/gamnit/flat/flat_core.nit:901,2--902,108