ffmpeg: Set default output format to hwaccel's native format

I think this is usually what the user wants - if they are using
an hwaccel, they probably want full hardware transcoding to
work.

ffmpeg_opt.c

@@ -708,6 +708,7 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
         char *discard_str = NULL;
         const AVClass *cc = avcodec_get_class();
         const AVOption *discard_opt = av_opt_find(&cc, "skip_frame", NULL, 0, 0);
+        enum AVPixelFormat default_pix_fmt = AV_PIX_FMT_NONE;
 
         if (!ist)
             exit_program(1);
@@ -805,6 +806,7 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
                     for (i = 0; hwaccels[i].name; i++) {
                         if (!strcmp(hwaccels[i].name, hwaccel)) {
                             ist->hwaccel_id = hwaccels[i].id;
+                            default_pix_fmt = hwaccels[i].pix_fmt;
                             break;
                         }
                     }
@@ -837,7 +839,7 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
                            "format: %s", hwaccel_output_format);
                 }
             } else {
-                ist->hwaccel_output_format = AV_PIX_FMT_NONE;
+                ist->hwaccel_output_format = default_pix_fmt;
             }
 
             ist->hwaccel_pix_fmt = AV_PIX_FMT_NONE;