GPencil: Refactor of Draw Engine, Vertex Paint and all internal functions

This commit is a full refactor of the grease pencil modules including Draw Engine, Modifiers, VFX, depsgraph update, improvements in operators and conversion of Sculpt and Weight paint tools to real brushes.

Also, a huge code cleanup has been done at all levels.

Thanks to @fclem for his work and yo @pepeland and @mendio for the testing and help in the development.

Differential Revision: https://developer.blender.org/D6293

source/blender/editors/object/object_edit.c

@@ -1370,7 +1370,8 @@ static const EnumPropertyItem *object_mode_set_itemsf(bContext *C,
                 OB_MODE_EDIT_GPENCIL,
                 OB_MODE_PAINT_GPENCIL,
                 OB_MODE_SCULPT_GPENCIL,
-                OB_MODE_WEIGHT_GPENCIL) &&
+                OB_MODE_WEIGHT_GPENCIL,
+                OB_MODE_VERTEX_GPENCIL) &&
            (ob->type == OB_GPENCIL)) ||
           (input->value == OB_MODE_OBJECT)) {
         RNA_enum_item_add(&item, &totitem, input);