fftools/ffmpeg_enc: avoid breaking exactly integer timestamps in vsync code

The code will currently add a small offset to avoid exact midpoints, but
this can cause inexact results when a float timestamp is exactly
representable as an integer.

Fixes off-by-one in the first frame duration in multiple FATE tests.

fftools/ffmpeg_enc.c

@@ -888,9 +888,11 @@ static double adjust_frame_pts_to_encoder_tb(OutputFile *of, OutputStream *ost,
     float_pts = av_rescale_q(frame->pts, filter_tb, tb) -
                 av_rescale_q(start_time, AV_TIME_BASE_Q, tb);
     float_pts /= 1 << extra_bits;
-    // avoid exact midoints to reduce the chance of rounding differences, this
+    // when float_pts is not exactly an integer,
+    // avoid exact midpoints to reduce the chance of rounding differences, this
     // can be removed in case the fps code is changed to work with integers
-    float_pts += FFSIGN(float_pts) * 1.0 / (1<<17);
+    if (float_pts != llrint(float_pts))
+        float_pts += FFSIGN(float_pts) * 1.0 / (1<<17);
 
     frame->pts = av_rescale_q(frame->pts, filter_tb, enc->time_base) -
                  av_rescale_q(start_time, AV_TIME_BASE_Q, enc->time_base);

tests/ref/fate/apng-osample

@@ -3,7 +3,7 @@
 #codec_id 0: rawvideo
 #dimensions 0: 116x135
 #sar 0: 0/1
-0,          0,          0,        2,    62640, 0x31eb581d
+0,          0,          0,        3,    62640, 0x31eb581d
 0,          3,          3,        3,    62640, 0x29e11b82
 0,          6,          6,        3,    62640, 0x207ed588
 0,          9,          9,        3,    62640, 0x3845c906

tests/ref/fate/film-cvid

@@ -3,7 +3,7 @@
 #codec_id 0: rawvideo
 #dimensions 0: 320x224
 #sar 0: 0/1
-0,          0,          0,        1,   215040, 0x067c5362
+0,          0,          0,        2,   215040, 0x067c5362
 0,          2,          2,        2,   215040, 0xd9eacb98
 0,          4,          4,        2,   215040, 0x3c8a4cbd
 0,          6,          6,        2,   215040, 0xbdf996e1

tests/ref/fate/filter-concat-vfr

@@ -8,7 +8,7 @@
 #codec_id 1: pcm_s16le
 #sample_rate 1: 44100
 #channel_layout_name 1: mono
-0,          0,          0,   199999,   230400, 0x88c4d19a
+0,          0,          0,   200000,   230400, 0x88c4d19a
 1,          0,          0,     1024,     2048, 0xb3f10192
 1,       1024,       1024,     1024,     2048, 0xb340fe4e
 1,       2048,       2048,     1024,     2048, 0x0a5f0111

tests/ref/fate/gif-gray

@@ -3,7 +3,7 @@
 #codec_id 0: rawvideo
 #dimensions 0: 480x360
 #sar 0: 0/1
-0,          0,          0,        4,   691200, 0xef6c0f3d
+0,          0,          0,        5,   691200, 0xef6c0f3d
 0,          5,          5,        2,   691200, 0xc18b32de
 0,          7,          7,        2,   691200, 0x2395a3d7
 0,          9,          9,        2,   691200, 0x81dc3cf2