Grease Pencil: Add LineArt modifier

This adds the LineArt grease pencil modifier.

It takes objects or collections as input and generates various grease
pencil lines from these objects with the help of the active scene
camera. For example it can generate contour lines, intersection lines
and crease lines to name a few.

This is really useful as artists can then use 3D meshes to automatically
generate grease pencil lines for characters, enviroments or other
visualization purposes.

These lines can then be baked and edited as regular grease pencil lines.

Reviewed By: Sebastian Parborg, Antonio Vazquez, Matias Mendiola

Differential Revision: http://developer.blender.org/D8758

source/blender/blenlib/BLI_math_base.h

@@ -117,6 +117,9 @@ MINLINE float sasqrt(float fac);
 MINLINE float interpf(float a, float b, float t);
 MINLINE double interpd(double a, double b, double t);
 
+MINLINE float ratiof(float min, float max, float pos);
+MINLINE double ratiod(double min, double max, double pos);
+
 /* NOTE: Compilers will upcast all types smaller than int to int when performing arithmetic
  * operation. */
 MINLINE int square_s(short a);