Implement custom depth pass rendering.

[Menotdan]

This PR implements the basics of a custom depth pass. It's missing a few things and also creates a weird dark artifact on meshes randomly when it's enabled.

I've implemented:

• Rendering of the depth pass
• Debug view for custom depth
• Lazy loading of the buffer
• Toggle in graphics settings
• Access to the custom depth buffer in materials

Not implemented:

• Custom stencil buffer reading/writing
• Skinned depth (yet, it's a pretty simple change)
• Fix for dark artifacts (this should be done before this PR is merged)

This PR also changed the GBuffer shader to allow for rendering the custom depth in a debug view. Here is the updated file to add to LFS:
GBuffer.zip