map chunk: normal recalculation for terrain is a bit broken

[bloerwald]

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@skarnproject reported on 2017-02-03

Since some time normal calculation algorithm for used for terrain brushes is a little bit different from Blizzard's one. To reproduce: edit terrain with zero speed and see changing shading on full chunk. Did not happen before.

[bloerwald]

Are you sure that calculation actually changed? I can't remember anyone touching that code in at least a year. Is it maybe that we now set

  gl.materialfv(GL_FRONT_AND_BACK, GL_SPECULAR, math::vector_4d (0.1f, 0.1f, 0.1f, 0.1f));

which makes this way more visible? -- okay, it isn't, without one still sees the shadows jumping a bit, God damn it.