Added IRenderPassHandler for dealing with render passes in RendererLivingEntity

Adds the IRenderPassHandler interface to deal with render passes in RendererLivingEntity

Functions:
- getRenderPassCount should return the desired amount of render passes and can be used as PreRenderPasses event.
- shouldRenderPass is basically the shouldRenderPass() function in RendererLivingEntity, return whether this pass should be rendered or not (-1 to cancel)
- renderPass is called before vanilla does it's render pass, so you can add your own render pass types, return false to cancel vanilla rendering.

This is a improved version of https://github.com/MinecraftForge/MinecraftForge/pull/721