x11grab: fixed next frame capture time calculation

The next frame time could slip, causing the frame rate to drop until
 frames were dropped. Now will capture at the next correct moment instead.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/x11grab.c

@@ -526,16 +526,11 @@ static int x11grab_read_packet(AVFormatContext *s1, AVPacket *pkt)
     int64_t curtime, delay;
     struct timespec ts;
 
-    /* Calculate the time of the next frame */
-    s->time_frame += INT64_C(1000000);
-
     /* wait based on the frame rate */
     for (;;) {
         curtime = av_gettime();
         delay   = s->time_frame * av_q2d(s->time_base) - curtime;
         if (delay <= 0) {
-            if (delay < INT64_C(-1000000) * av_q2d(s->time_base))
-                s->time_frame += INT64_C(1000000);
             break;
         }
         ts.tv_sec  = delay / 1000000;
@@ -543,6 +538,11 @@ static int x11grab_read_packet(AVFormatContext *s1, AVPacket *pkt)
         nanosleep(&ts, NULL);
     }
 
+    /* Calculate the time of the next frame */
+    do {
+      s->time_frame += INT64_C(1000000);
+    } while ((s->time_frame * av_q2d(s->time_base) - curtime) <= 0);
+
     av_init_packet(pkt);
     pkt->data = image->data;
     pkt->size = s->frame_size;