vcomp/lavc: per-codec help improvement

If both encoder and decoder of the same name is present, eg. jpeg2000,
print help for both encoder and decoder. Since for the decoder, options
are also printed with enc=<enc>:help, print both enc and dec at the
same time.

src/video_compress/libavcodec.cpp

@@ -182,7 +182,7 @@ static void setparam_jpeg(AVCodecContext *, struct setparam_param *);
 static void setparam_vp8_vp9(AVCodecContext *, struct setparam_param *);
 static void set_codec_thread_mode(AVCodecContext *codec_ctx, struct setparam_param *param);
 
-static void show_encoder_help(string const &name);
+static bool show_coder_help(string const &name, bool encoder = true);
 static void print_codec_supp_pix_fmts(const enum AVPixelFormat *first);
 void usage(bool full);
 static void cleanup(struct state_video_compress_libav *s);
@@ -487,7 +487,7 @@ handle_help(bool full, string const &req_encoder, string const &req_codec)
                 return;
         }
         if (!req_encoder.empty()) {
-                show_encoder_help(req_encoder);
+                show_coder_help(req_encoder);
                 return;
         }
         usage(full);
@@ -2068,19 +2068,30 @@ get_opt_default_value(const AVOption *opt)
         }
 }
 
-void show_encoder_help(string const &name) {
-        col() << "Options for " << SBOLD(name) << ":\n";
-        auto *codec = avcodec_find_encoder_by_name(name.c_str());
-        if (codec == nullptr) {
-                codec = avcodec_find_decoder_by_name(name.c_str());
+/// @aram encoder - if bool, print for both encoder and decoder
+static bool
+show_coder_help(string const &name, bool encoder)
+{
+        bool dec_found = true;
+        if (encoder) {
+                dec_found = show_coder_help(name, false);
         }
+        const auto *codec = encoder
+                                ? avcodec_find_encoder_by_name(name.c_str())
+                                : avcodec_find_decoder_by_name(name.c_str());
         if (codec == nullptr) {
-                LOG(LOG_LEVEL_ERROR) << MOD_NAME << "Unable to find encoder " << name << "!\n";
-                return;
+                if (!dec_found) {
+                        MSG(ERROR,
+                            "Unable to find neither encoder nor decoder %s!\n",
+                            name.c_str());
+                }
+                return false;
         }
+        col() << "Options for " << SBOLD(name) << " "
+              << (encoder ? "encoder" : "decoder") << ":\n";
         const auto *opt = codec->priv_class->option;
         if (opt == nullptr) {
-                return;
+                return true;
         }
         while (opt->name != nullptr) {
                 string default_val;
@@ -2133,6 +2144,8 @@ void show_encoder_help(string const &name) {
                         "\n\t  - use SVT AV1 in VBR mode with 3 Mbps (not "
                         "recommended at the moment, increases latency)\n");
         }
+        color_printf("\n");
+        return true;
 }
 
 static void setparam_vp8_vp9(AVCodecContext *codec_ctx, struct setparam_param *param)