Fix a race condition with KisMetaData::SchemaRegistry singleton
So I got this crash once. At the time I had three TIFF files in the recent file list. Since the recent files thumbnail icons are loaded in parallel for now, I suspect it happened to trigger a race condition when initializing the SchemaRegistry::instance()
singleton.
This change makes the singleton actually thread-safe by using a static local. Making an MR so that someone can sanity check my thinking.
Edited by Alvin Wong