fftools/ffmpeg: rename read_file() to avoid conflict with libass

libass defines a non-static read_file() symbol, which causes conflicts
with static linking.

fftools/ffmpeg.h

@@ -702,7 +702,7 @@ void remove_avoptions(AVDictionary **a, AVDictionary *b);
 void assert_avoptions(AVDictionary *m);
 
 void assert_file_overwrite(const char *filename);
-char *read_file(const char *filename);
+char *file_read(const char *filename);
 AVDictionary *strip_specifiers(AVDictionary *dict);
 const AVCodec *find_codec_or_die(const char *name, enum AVMediaType type, int encoder);
 int parse_and_set_vsync(const char *arg, int *vsync_var, int file_idx, int st_idx, int is_global);

fftools/ffmpeg_mux_init.c

@@ -380,7 +380,7 @@ static char *get_ost_filters(OptionsContext *o, AVFormatContext *oc,
     }
 
     if (ost->filters_script)
-        return read_file(ost->filters_script);
+        return file_read(ost->filters_script);
     else if (ost->filters)
         return av_strdup(ost->filters);
 
@@ -585,7 +585,7 @@ static OutputStream *new_video_stream(Muxer *mux, OptionsContext *o, int source_
                 av_dict_set(&ost->encoder_opts, "stats", logfilename, AV_DICT_DONT_OVERWRITE);
             } else {
                 if (video_enc->flags & AV_CODEC_FLAG_PASS2) {
-                    char  *logbuffer = read_file(logfilename);
+                    char  *logbuffer = file_read(logfilename);
 
                     if (!logbuffer) {
                         av_log(NULL, AV_LOG_FATAL, "Error reading log file '%s' for pass-2 encoding\n",

fftools/ffmpeg_opt.c

@@ -1300,7 +1300,7 @@ static int open_input_file(OptionsContext *o, const char *filename)
 }
 
 /* read file contents into a string */
-char *read_file(const char *filename)
+char *file_read(const char *filename)
 {
     AVIOContext *pb      = NULL;
     int ret = avio_open(&pb, filename, AVIO_FLAG_READ);
@@ -1725,7 +1725,7 @@ static int opt_filter_complex(void *optctx, const char *opt, const char *arg)
 static int opt_filter_complex_script(void *optctx, const char *opt, const char *arg)
 {
     FilterGraph *fg;
-    char *graph_desc = read_file(arg);
+    char *graph_desc = file_read(arg);
     if (!graph_desc)
         return AVERROR(EINVAL);