avfilter/vf_psnr: Add support for writing stats to stdout

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -8939,7 +8939,8 @@ The description of the accepted parameters follows.
 @table @option
 @item stats_file, f
 If specified the filter will use the named file to save the PSNR of
-each individual frame.
+each individual frame. When filename equals "-" the data is sent to
+standard output.
 @end table
 
 The file printed if @var{stats_file} is selected, contains a sequence of

libavfilter/vf_psnr.c

@@ -193,7 +193,9 @@ static av_cold int init(AVFilterContext *ctx)
     s->min_mse = +INFINITY;
     s->max_mse = -INFINITY;
 
-    if (s->stats_file_str) {
+    if (!strcmp(s->stats_file_str, "-")) {
+        s->stats_file = stdout;
+    } else if (s->stats_file_str) {
         s->stats_file = fopen(s->stats_file_str, "w");
         if (!s->stats_file) {
             int err = AVERROR(errno);
@@ -334,7 +336,7 @@ static av_cold void uninit(AVFilterContext *ctx)
 
     ff_dualinput_uninit(&s->dinput);
 
-    if (s->stats_file)
+    if (s->stats_file && s->stats_file != stdout)
         fclose(s->stats_file);
 }