This is a regression introduced by rBd8b00a3bf5c1 (Freestyle: memory
consumption optimization in stroke rendering).
The issue was caused by uninitialized MPoly::mat_nr values. Before the
stroke rendering optimization, individual Freestyle strokes were
represented by distinct mesh objects, and thus MPoly::mat_nr was left
unset (i.e., was always zero). Now that the stroke rendering optimization
has been done and mesh objects may represent multiple strokes of different
materials, MPoly::mat_nr had to be properly set to the material index that
refers to the material of the poly face.
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