Don't mark generated dummy frame output from mpeg1/2 decoder as keyframe.

We use this flag to tell when to assume decoded data is free
from artifacts after a seek. The mpeg1/2 decoder will generate
a dummy frame if decoding is started on a non keyframe. This
will sadly be output from decoder to player and displayed
as a gray screen.

libavcodec/mpegvideo.c

@@ -1114,6 +1114,7 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             /* Allocate a dummy frame */
             i= ff_find_unused_picture(s, 0);
             s->last_picture_ptr= &s->picture[i];
+            s->last_picture_ptr->key_frame = 0;
             if(ff_alloc_picture(s, s->last_picture_ptr, 0) < 0)
                 return -1;
             ff_thread_report_progress((AVFrame*)s->last_picture_ptr, INT_MAX, 0);
@@ -1123,6 +1124,7 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             /* Allocate a dummy frame */
             i= ff_find_unused_picture(s, 0);
             s->next_picture_ptr= &s->picture[i];
+            s->next_picture_ptr->key_frame = 0;
             if(ff_alloc_picture(s, s->next_picture_ptr, 0) < 0)
                 return -1;
             ff_thread_report_progress((AVFrame*)s->next_picture_ptr, INT_MAX, 0);