2.5: Animation playback without sync option was slightly slower

than expected time, even when it could keep up. Changed the WM
timer logic a bit to always target the next frame time exactly,

source/blender/windowmanager/WM_types.h

@@ -283,6 +283,8 @@ typedef struct wmTimer {
 	double delta;			/* time since previous step in seconds */
 	
 	double ltime;			/* internal, last time timer was activated */
+	double ntime;			/* internal, next time we want to activate the timer */
+	double stime;			/* internal, when the timer started */
 	int sleep;				/* internal, put timers to sleep when needed */
 } wmTimer;
 

source/blender/windowmanager/intern/wm_window.c

@@ -26,6 +26,7 @@
  * ***** END GPL LICENSE BLOCK *****
  */
 
+#include <math.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -690,12 +691,13 @@ static int wm_window_timer(const bContext *C)
 		wmTimer *wt;
 		for(wt= win->timers.first; wt; wt= wt->next) {
 			if(wt->sleep==0) {
-				if(wt->timestep < time - wt->ltime) {
+				if(time > wt->ntime) {
 					wmEvent event= *(win->eventstate);
 					
 					wt->delta= time - wt->ltime;
 					wt->duration += wt->delta;
 					wt->ltime= time;
+					wt->ntime= wt->stime + wt->timestep*ceil(wt->duration/wt->timestep);
 					
 					event.type= wt->event_type;
 					event.custom= EVT_DATA_TIMER;
@@ -790,6 +792,8 @@ wmTimer *WM_event_add_window_timer(wmWindow *win, int event_type, double timeste
 	
 	wt->event_type= event_type;
 	wt->ltime= PIL_check_seconds_timer();
+	wt->ntime= wt->ltime + timestep;
+	wt->stime= wt->ltime;
 	wt->timestep= timestep;
 	
 	BLI_addtail(&win->timers, wt);