Improved timer implementation

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

include/SDL_timer.h

@@ -41,104 +41,50 @@ extern "C" {
 #endif
 
 /**
- *  This is the OS scheduler timeslice, in milliseconds.
- */
-#define SDL_TIMESLICE		10
-
-/**
- *  This is the maximum resolution of the SDL timer on all platforms.
- */
-#define TIMER_RESOLUTION	10      /**< Experimentally determined */
-
-/**
- *  Get the number of milliseconds since the SDL library initialization.
+ * \brief Get the number of milliseconds since the SDL library initialization.
  *  
- *  Note that this value wraps if the program runs for more than ~49 days.
+ * \note This value wraps if the program runs for more than ~49 days.
  */
 extern DECLSPEC Uint32 SDLCALL SDL_GetTicks(void);
 
 /**
- *  Wait a specified number of milliseconds before returning.
+ * \brief Wait a specified number of milliseconds before returning.
  */
 extern DECLSPEC void SDLCALL SDL_Delay(Uint32 ms);
 
 /**
  *  Function prototype for the timer callback function.
- */
-typedef Uint32(SDLCALL * SDL_TimerCallback) (Uint32 interval);
-
-/**
- *  Set a callback to run after the specified number of milliseconds has
- *  elapsed. The callback function is passed the current timer interval
- *  and returns the next timer interval.  If the returned value is the 
- *  same as the one passed in, the periodic alarm continues, otherwise a
- *  new alarm is scheduled.  If the callback returns 0, the periodic alarm
- *  is cancelled.
- *  
- *  To cancel a currently running timer, call 
- *  \code SDL_SetTimer(0, NULL); \endcode
- *  
- *  The timer callback function may run in a different thread than your
- *  main code, and so shouldn't call any functions from within itself.
- *  
- *  The maximum resolution of this timer is 10 ms, which means that if
- *  you request a 16 ms timer, your callback will run approximately 20 ms
- *  later on an unloaded system.  If you wanted to set a flag signaling
- *  a frame update at 30 frames per second (every 33 ms), you might set a 
- *  timer for 30 ms:
- *  \code
- *    SDL_SetTimer((33/10)*10, flag_update);
- *  \endcode
- *  
- *  If you use this function, you need to pass ::SDL_INIT_TIMER to SDL_Init().
- *  
- *  Under UNIX, you should not use raise or use SIGALRM and this function
- *  in the same program, as it is implemented using setitimer().  You also
- *  should not use this function in multi-threaded applications as signals
- *  to multi-threaded apps have undefined behavior in some implementations.
- *  
- *  \return 0 if successful, or -1 if there was an error.
- */
-extern DECLSPEC int SDLCALL SDL_SetTimer(Uint32 interval,
-                                         SDL_TimerCallback callback);
-
-/**
- *  \name Peter timers
- *  New timer API, supports multiple timers
- *  Written by Stephane Peter <megastep@lokigames.com>
- */
-/*@{*/
-
-/**
- *  Function prototype for the new timer callback function.
  *  
  *  The callback function is passed the current timer interval and returns
  *  the next timer interval.  If the returned value is the same as the one
  *  passed in, the periodic alarm continues, otherwise a new alarm is
  *  scheduled.  If the callback returns 0, the periodic alarm is cancelled.
  */
-typedef Uint32(SDLCALL * SDL_NewTimerCallback) (Uint32 interval, void *param);
+typedef Uint32 (SDLCALL * SDL_TimerCallback) (Uint32 interval, void *param);
 
 /**
- *  Definition of the timer ID type.
+ * Definition of the timer ID type.
  */
-typedef struct _SDL_TimerID *SDL_TimerID;
+typedef int SDL_TimerID;
 
 /**
- *  Add a new timer to the pool of timers already running.
- *  \return A timer ID, or NULL when an error occurs.
+ * \brief Add a new timer to the pool of timers already running.
+ *
+ * \return A timer ID, or NULL when an error occurs.
  */
 extern DECLSPEC SDL_TimerID SDLCALL SDL_AddTimer(Uint32 interval,
-                                                 SDL_NewTimerCallback
-                                                 callback, void *param);
+                                                 SDL_TimerCallback callback,
+                                                 void *param);
 
 /**
- *  Remove one of the multiple timers knowing its ID.
- *  \return A boolean value indicating success or failure.
+ * \brief Remove a timer knowing its ID.
+ *
+ * \return A boolean value indicating success or failure.
+ *
+ * \warning It is not safe to remove a timer multiple times.
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_RemoveTimer(SDL_TimerID t);
 
-/*@}*//*Peter timers*/
 
 /* Ends C function definitions when using C++ */
 #ifdef __cplusplus