Blender Git Loki

Fix grease pencil cache update problems when change material properties

Since commit f84956738ba4e97f30553a675b643f6648afce1d 6/07/2018 12:33, the update of the materials was not working because depsgraph only update the current ID and not all related.

This fix the issue but need a review.

@aligorith Could you take a look and check if this is the best solution, I'm not sure about the fix, maybe we could add a relationship between material and datablock when create a new material.