Blender Git Loki

GPencil: Fix unreported problem when use Onion Skin in several windows

The onion skin was nos displayed in secondary screens when disable the Onion switch or the Overlay in the main window.

Added a check to verify if the main overlay and onion switches are enabled in any screen in order to generate the cache data.

This is required to generate the onion skin and limit the times the cache is updated because the cache is generated only in the first screen and if the first screen has the onion disabled the cache for onion skin is not generated. The loop adds time, but always is faster than regenerate the cache all the times.

Reviewed By: mendio, pepeland

Differential Revision: https://developer.blender.org/D6049