Add streamable file format options for mp4, mov

Add 2 new multiplexing presets for mp4, mov files:
mp4_faststart
mp4_fragmented

The Preset usage from openshot-qt export.py as follows (example):
w.SetOption(openshot.VIDEO_STREAM, "muxing_preset", "mp4_faststart")

YouTube suggest to use streamable file formats to process the uploading
videos faster (by starting its processing when upload not yet complete)
MP4, MOV files export requires additional dictionary keys to be set in
this case.

Author: SuslikV

src/FFmpegWriter.cpp

@@ -38,6 +38,9 @@ using namespace openshot;
 #pragma message "You are compiling only with software encode"
 #endif
 
+// Multiplexer parameters temporary storage
+AVDictionary *mux_dict = NULL;
+
 #if IS_FFMPEG_3_2
 int hw_en_on = 1;					// Is set in UI
 int hw_en_supported = 0;	// Is set by FFmpegWriter
@@ -464,6 +467,17 @@ void FFmpegWriter::SetOption(StreamType stream, string name, string value) {
 
 		ZmqLogger::Instance()->AppendDebugMethod("FFmpegWriter::SetOption (" + (string)name + ")", "stream == VIDEO_STREAM", stream == VIDEO_STREAM, "", -1, "", -1, "", -1, "", -1, "", -1);
 
+	// Muxing dictionary is not part of the codec context.
+	// Just reusing SetOption function to set popular multiplexing presets.
+	} else if (name == "muxing_preset") {
+		if (value == "mp4_faststart") {
+			// 'moov' box to the beginning; only for MOV, MP4
+			av_dict_set(&mux_dict, "movflags", "faststart", 0);
+		} else if (value == "mp4_fragmented") {
+			// write selfcontained fragmented file, minimum length of the fragment 8 sec; only for MOV, MP4
+			av_dict_set(&mux_dict, "movflags", "frag_keyframe", 0);
+			av_dict_set(&mux_dict, "min_frag_duration", "8000000", 0);
+			}
 	} else {
 		throw InvalidOptions("The option is not valid for this codec.", path);
 	}
@@ -511,17 +525,29 @@ void FFmpegWriter::WriteHeader() {
 	snprintf(oc->AV_FILENAME, sizeof(oc->AV_FILENAME), "%s", path.c_str());
 
 	// Write the stream header, if any
-	// TODO: add avoptions / parameters instead of NULL
 
 	// Add general metadata (if any)
 	for (std::map<string, string>::iterator iter = info.metadata.begin(); iter != info.metadata.end(); ++iter) {
 		av_dict_set(&oc->metadata, iter->first.c_str(), iter->second.c_str(), 0);
 	}
 
-	if (avformat_write_header(oc, NULL) != 0) {
+	// Set multiplexing parameters
+	AVDictionary *dict = NULL;
+
+	bool is_mp4 = strcmp(oc->oformat->name, "mp4");
+	bool is_mov = strcmp(oc->oformat->name, "mov");
+	// Set dictionary preset only for MP4 and MOV files
+	if (is_mp4 || is_mov)
+		av_dict_copy(&dict, mux_dict, 0);
+
+	if (avformat_write_header(oc, &dict) != 0) {
 		throw InvalidFile("Could not write header to file.", path);
 	};
 
+	// Free multiplexing dictionaries sets
+	if (dict) av_dict_free(&dict);
+	if (mux_dict) av_dict_free(&mux_dict);
+
 	// Mark as 'written'
 	write_header = true;