Add Depth output to Normal Map node

[myaaaaaaaaa]

Adds a "Depth" output whose scale is linked to the strength of the normal map, to streamline height-only workflows.

Intended usage:

The Normal Map node being proposed by this pull request:

[RodZill4]

It's a neat trick, but multiplying depth might be a bad idea for high values of Strength. If anything, I'd remove the multiplier node. Correlation between normal map and height map depends on the model anyway.

[myaaaaaaaaa]

Thanks for the feedback! The idea is that the exact ratio between normal and depth can be adjusted with the material's depth_scale parameter, which can be set once based on the model's uv scale. Afterwards, the Strength parameter would be used to tweak heightmap scale with correct normals automatically applied, which I find especially useful for quick experimenting.

Workflows where normal and depth are less correlated would of course calculate depth from height themselves, and would not use this new Depth output.