conf: make delayed rendering timeouts configurable

This adds an undocumented 'tweak' section to footrc, with two new
options:

* delayed-render-lower
* delayed-render-upper

Both takes an integer value, representing the lower/upper timeout
values (in nano seconds) for delayed rendering.

config.h

@@ -73,6 +73,11 @@ struct config {
     char *server_socket_path;
     bool presentation_timings;
     bool hold_at_exit;
+
+    struct {
+        uint64_t delayed_render_lower_ns;
+        uint64_t delayed_render_upper_ns;
+    } tweak;
 };
 
 bool config_load(struct config *conf, const char *path);