If I remember correctly, this was needed during development of
89e3ba4e25, but not anymore. Now the face normals are used
if faces are sharp. And if there is a mixture of sharp and smooth faces,
then normals_domain will be Corner anyway.
In a basic test with a 16 million face grid, this saved 30ms every draw
extraction. It also saves about 12 bytes per corner which was cached on
the mesh before.
There's a chance I'm missing something and this will require changes
elsewhere, but it's working well in my testing.
c4446d7924