BLI: support disabling task isolation in task pool

Under some circumstances using task isolation can cause deadlocks.
Previously, our task pool implementation would run all tasks in an
isolated region. Now using task isolation is optional and can be
turned on/off for individual task pools.

Task pools that spawn new tasks recursively should never enable
task isolation. There is a new check that finds these cases at runtime.
Right now this check is disabled, so that this commit is a pure refactor.
It will be enabled in an upcoming commit.

This fixes T88598.

Differential Revision: https://developer.blender.org/D11415

source/blender/blenkernel/intern/lib_override.c

@@ -2335,7 +2335,8 @@ bool BKE_lib_override_library_main_operations_create(Main *bmain, const bool for
   }
 
   struct LibOverrideOpCreateData create_pool_data = {.bmain = bmain, .changed = false};
-  TaskPool *task_pool = BLI_task_pool_create(&create_pool_data, TASK_PRIORITY_HIGH);
+  TaskPool *task_pool = BLI_task_pool_create(
+      &create_pool_data, TASK_PRIORITY_HIGH, TASK_ISOLATION_ON);
 
   FOREACH_MAIN_ID_BEGIN (bmain, id) {
     if (!ID_IS_LINKED(id) && ID_IS_OVERRIDE_LIBRARY_REAL(id) &&