https://github.com/flutter/flutter/pull/162671/changes added a mechanism for setting the threading policy. We should follow that pattern to add a switch to allow people to opt into impeller.

This should not be implemented until integration tests for impeller linux have been expanded.

g_autoptr(FlDartProject) project = fl_dart_project_new();
fl_dart_project_set_ui_thread_policy(project,
  FL_UI_THREAD_POLICY_RUN_ON_SEPARATE_THREAD);