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Cleanup, LineArt: Fix MSVC compiler warnings.

Currently 3 of the printf lines use the format specifier "%lu" for data
of type size_t instead of "%zu". While this works, this creates compiler
warnings.

This diff fixes those warnings by using the correct format specifier.
Tested using MSVC, but should be correct on any compliant compiler.

Reviewed By: Sebastian Parborg

Differential Revision: https://developer.blender.org/D10761