* Simplify SymbolLayout; it never needs to care about more than one SpriteAtlas. * Move the reference from SymbolLayer::Impl to SymbolBucket. This is a prerequisite for making layer Impls immutable.
* Simplify SymbolLayout; it never needs to care about more than one SpriteAtlas. * Move the reference from SymbolLayer::Impl to SymbolBucket. This is a prerequisite for making layer Impls immutable.