Fix worker not starting after plugin reload (mac)

[amyinorbit]

• On macOS, it seems like global storage isn’t re-initialised when plugins are reloaded, leading to worker_shutdown staying true, and the worker thread exiting immediately. Initialised the variable to false in xtcas_init() to prevent this.

[amyinorbit]

Currently investigating similar issues with some VSI dataref getting checked before being resolved in similar circumstances, will un-draft when I've figured that out.