`LayerBuilder::for_each_tiles` won't create entities as needed

[BrettWitty]

I'm attempting to set all tiles of a layer to be GPUAnimated. I'm using bevy v0.6.1 and bevy_ecs_tilemap v0.5.0.

I assumed I could use set_all or fill to set the initial frame, then use for_each_tiles to add in the GPUAnimated component. However set_all and fill do not create tile entities prior to layer building. The animation example in the repo uses get_tile_entity which does create an entity if it doesn't exist, but you need to explicitly loop through all the TilePos.

I found this confusing.