render: keep lock while pushing dirty rows to worker queue

Instead of locking the queue for each dirty row we append, and
signaling a condition variable, just keep the lock while going through
the visible rows.

Release the lock once done.

Since we take the lock *before* posting the 'start' semaphore, all
workers will be waiting for the lock to be released.

Then, one at a time they'll get the lock and pick a row to
render. The queue will never get empty - when all rows have been
rendered, each worker will pick a 'frame done' "job" from the queue,
and break the rendering loop.

terminal.h

@@ -369,7 +369,6 @@ struct terminal {
             size_t count;
             sem_t start;
             sem_t done;
-            cnd_t cond;
             mtx_t lock;
             tll(int) queue;
             thrd_t *threads;