Render Graph as Systems

Bevy's RenderGraph architecture has been replaced with schedules. Render passes are now regular systems that run in the Core3d, Core2d, or custom rendering schedules.

The render graph was originally designed when Bevy's ECS was less mature. As our APIs have evolved, Schedule has become capable of expressing the core render graph pattern. This change lets rendering better leverage familiar Bevy patterns, as well as opening the door to new optimizations and ECS-driven features in the future, including plans for parallel command buffer recording.