render worker context: allocate, and let worker threads free

Since we now initialize the worker threads from term_init(), which
returns before the threads terminate, we can no longer use
stack-allocated worker contexts.

We _could_ put them in the terminal struct. But a simpler solution is
to allocate them in term_init(), and let the threads free them when
they don't need them anymore.

render.c

@@ -374,6 +374,7 @@ render_worker_thread(void *_ctx)
     struct render_worker_context *ctx = _ctx;
     struct terminal *term = ctx->term;
     const int my_id = ctx->my_id;
+    free(ctx);
 
     char proc_title[16];
     snprintf(proc_title, sizeof(proc_title), "foot:render:%d", my_id);