Add function to find nearest keyframe to time

Can be tested using existing json files eg:

    import random
    for source in json_in["sources"]:
        time = round(random.uniform(30, 600), 2)
        print("Finding nearest keyframe to {} in {}".format(
            time, source["filepath"]))
        print("Nearest keyframe: {}-{}".format(
            time, nearest_keyframe(source["filepath"], time)))

    import sys
    sys.exit(0)

efficientjson.py

@@ -222,6 +222,52 @@ def process_highlights(jsonfile):
 
         return outputfile
 
+    def nearest_keyframe(videofile=None, checktime=0):
+        """Find nearest keyframe to checktime using ffprobe
+
+        see eg https://superuser.com/a/1426307"""
+        import subprocess
+
+        if checktime > 15:
+            skiptime = checktime - 15
+        else:
+            skiptime = checktime
+
+        frames = []
+
+        ffprobe_args = ["ffprobe", "-read_intervals", str(skiptime),
+                        "-select_streams", "v",
+                        "-skip_frame", "nokey",  # skip all non keyframes
+                        "-show_frames", "-show_entries",
+                        "frame=pkt_pts_time",
+                        videofile]
+
+        # complete = subprocess.run(ffprobe_args, capture_output=True)
+        proc = subprocess.Popen(ffprobe_args,
+                                stdout=subprocess.PIPE,
+                                stderr=subprocess.DEVNULL,
+                                universal_newlines=True)
+        while not proc.poll():
+            line = proc.stdout.readline()
+            if "pkt_pts_time" in line:
+                frametime = float(line.split("=")[1])
+                frames.append(frametime)
+            if len(frames) > 20:
+                proc.kill()
+
+        # if "stdout" in complete:
+        #    for line in complete["stdout"].split("\n"):
+        #        if "pkt_pts_time" in line:
+        #            frames.extend(line.split("=")[1])
+
+        # get closest value, see https://stackoverflow.com/a/12141207
+        keyframetime = min(frames, key=lambda x: abs(x-checktime))
+
+        return str(round(keyframetime-0.1, 2))
+        # we return the time just before it
+        # so that ffmpeg seeks to that when
+        # cutting
+
     # Start
     import os