Merge pull request #4 from raustin9/raustin9-3i-shader-map

use shader system in material shaders

refactor render pass nodes to use new RenderPassContext rather than gfx::Context directly

able to get shader features per material in renderpass execute

add shader map and create gbuffer shader with it

create shader feature utilities and shader registry

fix image links

update image

update images

fix bug in diffuse color pipeline binding

use pipeline manager and use requirements to get pipeline at runtime

update examples in readme

update image size in readme

use lights in alpha

use lights in opaque pass