NME: multiple changes to support ray marching in the NME

[Popov72]

The main changes are that some blocks can be configured to generate their code in the fragments shader only (Lights, PBRMetallicRoughness, Reflection).

New blocks have also been added: FragDepthBlock to write fragment depth, ShadowMapBlock to generate depth in a shadow map.

The ShadowDepthWrapper has also been updated to work with materials using a ShadowMapBlock.

A couple of helper blocks have also been created: TriPlanarBlock and BiPlanarBlock for sampling a texture with a tri/bi planar mapping (ray marching does not usually produce uvs, so we need to be able to sample a texture with a position+normal instead).

Note that we haven't created a ray marching node itself, but it's now possible for users to create one. In the PG demo below, we have implemented a very basic one with a CustomBlock.

Here is a PG that uses all these new features:

https://playground.babylonjs.com/#M3QR7E#34

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[sebavan]

Looks good to me only minor comments and mostly the same 3 in various places :-)