Fix terrain vertex normals

[amelentev]

It seems there is some bug in the vertex normal calculation, getGridNormal. The normal vector for the center of a tile does not correspond to near normals. You can see it if you draw only light in terrain_combined_medium.frag:

I'm looking into the issue.
For now, I have one workaround - to average tile normals for the center point. Since the center point is the average of tile points. Looks better:

[MaNGusT-]

As I thought, for some situations average normals work better. With such smooth normals ssao will look super good and natural.

Average vertex normals is classic method to smooth surface BUT they work really bad when angle between face normals is greater than 60* and completely wrong when greater than 90*. In these situations weightened vertex normals are strongly recommended. Probably it's also mesh-dependent.

[MaNGusT-]

We also have "color binding" effect since sRGB convertion (from/to) is a bit broken. So you probably can't get perfect results.
It's visible on your screenshots but clearly visible if move the camera under water to look at bottom texture.