options: enable handling --no-hwdec as --hwdec=no

Reusing M_OPT_ALLOW_NO has the side-effect of applying to --ab-loop-[ab],
which makes sense.

options/m_config_frontend.c

@@ -662,10 +662,13 @@ static struct m_config_option *m_config_mogrify_cli_opt(struct m_config *config,
     bstr no_name = *name;
     if (!co && bstr_eatstart0(&no_name, "no-")) {
         co = m_config_get_co(config, no_name);
+        if (!co)
+            return NULL;
 
         // Not all choice types have this value - if they don't, then parsing
         // them will simply result in an error. Good enough.
-        if (!co || !(co->opt->type->flags & M_OPT_TYPE_CHOICE))
+        if (!(co->opt->type->flags & M_OPT_TYPE_CHOICE) &&
+            !(co->opt->flags & M_OPT_ALLOW_NO))
             return NULL;
 
         *name = no_name;

options/m_option.h

@@ -468,6 +468,8 @@ char *format_file_size(int64_t size);
 #define M_OPT_DEFAULT_NAN       (1 << 26)
 
 // type time: string "no" maps to MP_NOPTS_VALUE (if unset, NOPTS is rejected)
+// and
+// parsing: "--no-opt" is parsed as "--opt=no"
 #define M_OPT_ALLOW_NO          (1 << 27)
 
 // type channels: disallow "auto" (still accept ""), limit list to at most 1 item.

video/decode/vd_lavc.c

@@ -124,7 +124,7 @@ const struct m_sub_options vd_lavc_conf = {
         {"vd-apply-cropping", OPT_BOOL(apply_cropping)},
         {"hwdec", OPT_STRINGLIST(hwdec_api),
             .help = hwdec_opt_help,
-            .flags = M_OPT_OPTIONAL_PARAM | UPDATE_HWDEC},
+            .flags = M_OPT_OPTIONAL_PARAM | M_OPT_ALLOW_NO | UPDATE_HWDEC},
         {"hwdec-codecs", OPT_STRING(hwdec_codecs)},
         {"hwdec-image-format", OPT_IMAGEFORMAT(hwdec_image_format)},
         {"hwdec-extra-frames", OPT_INT(hwdec_extra_frames), M_RANGE(0, 256)},