This is useful for platforms that do not have support for threading or
programs that do not want to use an extra thread for resource
management and would rather process jobs manually.

When configuring the resource manager to not use threading, the
MA_RESOURCE_MANAGER_FLAG_NO_THREADING flag must be set in the config.
This implicitly enables the MA_RESOURCE_MANAGER_FLAG_NON_BLOCKING flag
because it requires programs to manually call
ma_resource_manager_process_next_job(), and since it's assumed that
won't ever be called from another thread, you'd never want that to be
blocking.

This sets up a framework for getting the resource manager working with
Emscripten.