Add RenderPass support?

Currently a pipeline in amber stores a VkRenderPass and then creates a Pipeline in that pass when we execute the draw call. Effectively, each pipeline has an individual renderpass.

It is possible for multiple pipelines to be in a given renderpass. Should we expose this feature through Amber or have it tested in other ways?

Sketch of Amberscript:

```
RENDERPASS pass
BIND BUFFER framebuffer AS color LOCATION 0
END

BUFFER framebuffer FORMAT B8G8R8A8_UNORM

PIPELINE graphics my_pipeline
ATTACH vert_shader
ATTACH frag_shader

RENDERPASS pass
END

PIPELINE graphics my_pipeline2
ATTACH vert_shader
ATTACH frag_shader

RENDERPASS pass
END
```