GPencil: New Trace images using Potrace

This patch adds a new operator to convert a black and white image into
grease pencil strokes.

If the image is not B/W, an internal conversion is done.

This is the first operator using Potrace, but we expect to add more features in next Blender versions.

Reviewed By: HooglyBoogly

Maniphest Tasks: T79877

Differential Revision: https://developer.blender.org/D8951

source/blender/python/intern/bpy_app_build_options.c

@@ -61,6 +61,7 @@ static PyStructSequence_Field app_builtopts_info_fields[] = {
     {"usd", NULL},
     {"fluid", NULL},
     {"xr_openxr", NULL},
+    {"potrace", NULL},
     {NULL},
 };
 
@@ -282,6 +283,12 @@ static PyObject *make_builtopts_info(void)
   SetObjIncref(Py_False);
 #endif
 
+#ifdef WITH_POTRACE
+  SetObjIncref(Py_True);
+#else
+  SetObjIncref(Py_False);
+#endif
+
 #undef SetObjIncref
 
   return builtopts_info;