What's new:
- New metal building block: 'FullScreenQuad'.
- Added 'setup' and 'startup' functions in 'MetalBuildingBlock' and 'MetalBuilderView'.
- Added ‘Renderable’ protocol for 'MetalBuilderComponent' ('Render' component conforms to it).
- Added ‘renderableData’ to 'MetalBuildingBlock' and the initializer of 'Render'.
- Added 'stencilAttachment' to ‘RenderableData’.
- Fixes and refactorings.
- Updated documentation.