Issue was caused by resetting face's mat_nr to zero if there's no material
map sent to ConvertCSGDescriptorsToDerivedMesh. In case of boolean modifier
we can't use such map because we can't affect on materials present in object.
So the only way which can give reasonable result is:
- Dot change mat_nr for faces from left operand (they should be fine, because
materials aren't deleting by modifier)
- For faces from right operand check if needed material exists in left operand
and if so, use it's index as new mat_nr.
- If there are materials in right operand which doesn't exist in left operand,
they'll be changed to first material from left operand.
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