Currently, ResourceGenerationContext is being converted from a record with an internal constructor into an abstract class which can be inherited. In a future breaking change, ResourceGenerationContext will be converted into an interface instead, and ResourceCache#getContext will be renamed makeContext and made abstract.

At the same time, I need to change it to look more like a ResourceManager and remove that parameter from DynamicSpriteSource#getSources

Completed with current state of #71