preparser: add the possibility to preparse on an external process

* Add a new backend to preform preparsing by sending preparser request
   to an external process and wait for the preparsed media in a
   preparser responce format.

 * Use the new preparser with external process inside the playlist and
   the medialib thumbnailer.

src/Makefile.am

@@ -278,6 +278,7 @@ libvlccore_la_SOURCES = \
 	preparser/internal.c \
 	preparser/preparser.c \
 	preparser/preparser.h \
+	preparser/external.c \
 	input/item.c \
 	input/access.c \
 	clock/clock_internal.c \

src/meson.build

@@ -128,6 +128,7 @@ libvlccore_sources_base = files(
     'playlist/sort.c',
     'preparser/art.c',
     'preparser/art.h',
+    'preparser/external.c',
     'preparser/fetcher.c',
     'preparser/fetcher.h',
     'preparser/ipc.c',

src/preparser/preparser.c

@@ -41,7 +41,7 @@ vlc_preparser_t *vlc_preparser_New(vlc_object_t *obj,
         return NULL;
     }
     if (cfg->external_process) {
-        vlc_assert_unreachable();
+        preparser->sys = vlc_preparser_external_New(preparser, obj, cfg);
     } else {
         preparser->sys = vlc_preparser_internal_New(preparser, obj, cfg);
     }

src/preparser/preparser.h

@@ -59,6 +59,10 @@ void *vlc_preparser_internal_New(vlc_preparser_t *preparser,
                                  vlc_object_t *parent,
                                  const struct vlc_preparser_cfg *cfg);
 
+void *vlc_preparser_external_New(vlc_preparser_t *preparser,
+                                 vlc_object_t *parent,
+                                 const struct vlc_preparser_cfg *cfg);
+
 /* Preparser Request */
 struct vlc_preparser_req_operations {
     /* Called by `vlc_preparser_req_GetItem`. */