update terrainMaterials.glsl to fix diagonal lines on modded ores

[tovc]

gonna be perfectly honest, I don't have a fucking clue what this does differently

someone more intelligent than I am should probably test this before merging

before:

after:

[tovc]

ok, so this does cause some ores, namely those from Draconic Evolution, to look slightly darker

before:

after:

[tovc]

ok turns out I have no idea what I'm doing and this working is just a coincidence