GPencil: Use default material when slot material is empty

When a material slot is empty, the default material is used.

The default color used is Gray to get a good contrast in dense scenes with dark and white background.

Reviewers: brecht, dfelinto, mendio, pepeland

Differential Revision: https://developer.blender.org/D5625

source/blender/windowmanager/intern/wm_init_exit.c

@@ -524,6 +524,7 @@ void WM_exit_ex(bContext *C, const bool do_python)
 
   BKE_addon_pref_type_free();
   BKE_keyconfig_pref_type_free();
+  BKE_material_gpencil_default_free();
 
   wm_operatortype_free();
   wm_dropbox_free();