Blender Git Loki

Fix: GP modifiers not updating when properties changed

Added back an explicit call to BKE_gpencil_batch_cache_dirty()
in the rna update callback for GPencil modifier properties.

However, instead of polluting the general rna_Modifier_update(),
this time we do this in a special wrapper/update callback for
dedicated GP modifier usage.

I'm unsure why the depsgraph isn't properly updating this case
- there's probably a missing relation between modifier properties
and GP ubereval (which calls the tagging), but where?!