Dispatch atleast one callback, blocking until the callback is dispatched.
EVS_DISPATCH_ALL
Dispatch all waiting callbacks without blocking to wait for any callbacks.
EVS_DISPATCH_BLOCKING
Dispatch all callbacks blocking indefinately. This is used in a threaded
program where a thread is created, and then evs_dispatch() is called immediately
from the created thread to execute callbacks.
RETURN VALUE
This call returns the EVS_OK value if successful, otherwise an error is returned.