Fix T43624: Freestyle uses wrong colour on second Line Set with textured lines in...

Fix T43624: Freestyle uses wrong colour on second Line Set with textured lines in Cycles.

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.