Fix inacurate frame timing

All time references are now kept in clock tick to avoid imprecision
due to unit changes. s_last_frame is now updated only when we
effectively encode a frame. s_last_frame and b_start_dumping are now
used in a way similar to the Windows implementation.

Source/Core/VideoCommon/AVIDump.cpp

@@ -335,7 +335,7 @@ static int s_height;
 static int s_size;
 static u64 s_last_frame;
 bool b_start_dumping = false;
-static s64 s_last_pts;
+static u64 s_last_pts;
 
 static void InitAVCodec()
 {
@@ -353,7 +353,7 @@ bool AVIDump::Start(int w, int h)
 	s_height = h;
 
 	s_last_frame = CoreTiming::GetTicks();
-	s_last_pts = AV_NOPTS_VALUE;
+	s_last_pts = 0;
 
 	InitAVCodec();
 	bool success = CreateFile();
@@ -445,15 +445,14 @@ void AVIDump::AddFrame(const u8* data, int width, int height)
 	int got_packet = 0;
 	int error = 0;
 	u64 delta = CoreTiming::GetTicks() - s_last_frame;
-	if (s_last_pts != AV_NOPTS_VALUE)
-		s_scaled_frame->pts = s_last_pts;
-	else
-		s_scaled_frame->pts = 0;
-	s_scaled_frame->pts += (delta * s_stream->codec->time_base.den) / SystemTimers::GetTicksPerSecond();
-	s_last_frame += delta;
-	if (s_scaled_frame->pts != s_last_pts || s_last_pts == AV_NOPTS_VALUE)
+	u64 pts_in_ticks = s_last_pts + delta;
+	s64 last_pts = (s_last_pts * s_stream->codec->time_base.den) / SystemTimers::GetTicksPerSecond();
+	s_scaled_frame->pts = (pts_in_ticks * s_stream->codec->time_base.den) / SystemTimers::GetTicksPerSecond();
+	if (s_scaled_frame->pts != last_pts || !b_start_dumping)
 	{
-		s_last_pts = s_scaled_frame->pts;
+		s_last_frame = CoreTiming::GetTicks();
+		s_last_pts = pts_in_ticks;
+		b_start_dumping = true;
 		error = avcodec_encode_video2(s_stream->codec, &pkt, s_scaled_frame, &got_packet);
 	}
 	while (!error && got_packet)