Fixes #110

Cleaned up API. Also added some new types to better encapsulate logic
and layout information.

- [x] `GpuImage` requires `texture_format` now.
- [ ] `ActiveCameras` was removed
  - https://github.com/bevyengine/bevy/commit/bf6de8962287050369cd98605490bdd7770c87b4
- [x] `CachedPipelineId` is now `CachedRenderPipelineId` and renamed `RenderPipelineCache` to `PipelineCache`
  - https://github.com/bevyengine/bevy/pull/3979

Cleaned up API. Also added some new types to better encapsulate logic
and layout information.