Introduce non-droppable control messages

Control messages are queued from the main thread and sent to the device
from a separate thread.

When the queue is full, messages are just dropped. This avoids to
accumulate too much delay between the client and the device in case of
network issue.

However, some messages should not be dropped: for example, dropping a
UHID_CREATE message would make invalid all further UHID_INPUT messages.
Therefore, mark these messages as non-droppable.

A non-droppable event is queued anyway (resizing the queue if
necessary, unless the allocation fails).

app/src/control_msg.c

@@ -278,6 +278,13 @@ sc_control_msg_log(const struct sc_control_msg *msg) {
     }
 }
 
+bool
+sc_control_msg_is_droppable(const struct sc_control_msg *msg) {
+    // Cannot drop UHID_CREATE messages, because it would cause all further
+    // UHID_INPUT messages for this device to be invalid
+    return msg->type != SC_CONTROL_MSG_TYPE_UHID_CREATE;
+}
+
 void
 sc_control_msg_destroy(struct sc_control_msg *msg) {
     switch (msg->type) {