The following patch fixes some of the bitrot for the Nintendo DS port.
The support is still basic at the moment, but it allows to run the "general"
test under the current head of tree (parent: 5269:11bd1585efb5 tip).
Most of the patch is mine, but I integrated a couple changes that John
Magnotti posted on Feb 1st.
The new timer model is formalized as using a separate thread to handle timer callbacks. This was the case on almost every platform before, but it's now a requirement, and simplifies the implementation and makes it perform consistently across platforms.
Goals:
* Minimize timer thread blocking
* Dispatch timers as accurately as possible
* SDL_AddTimer() and SDL_RemoveTimer() are completely threadsafe
* SDL_RemoveTimer() doesn't crash with a timer that's expired or removed
