Add deferred shading example

Add deferred shading example.
It demonstrates the use of render targets and uniform buffers.

It has a warning I don't understand:
```
examples/deferred/main.rs:131:1: 131:27 warning: unused import, #[warn(unused_imports)] on by default
examples/deferred/main.rs:131 #[shader_param(BlitBatch)]
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of #[shader_param]
examples/deferred/main.rs:131:1: 131:27 note: expansion site
```