filter: add a function to load filters

The activate function should match exactly vlc_filter_open, rather than use
a generic vlc_object_t loader. Then we don't have any dirty cast.

The code is similar to module_need, with better typing.

It's not possible to inline this because we need to call vlc_objres_clear()
for each module loaded unsuccessfully. It's not a public API.

include/vlc_filter.h

@@ -243,6 +243,9 @@ static inline void filter_Close( filter_t *p_filter )
         p_filter->ops->close( p_filter );
 }
 
+VLC_API module_t *vlc_filter_LoadModule(filter_t *, const char *cap,
+                                        const char *name, bool strict);
+
 /**
  * This function will return a new picture usable by p_filter as an output
  * buffer. You have to release it using picture_Release or by returning

src/libvlccore.sym

@@ -135,6 +135,7 @@ filter_chain_ForEach
 filter_ConfigureBlend
 filter_DeleteBlend
 filter_NewBlend
+vlc_filter_LoadModule
 FromCharset
 vlc_find_iso639
 vlc_http_auth_Init

src/misc/filter_chain.c

@@ -33,6 +33,33 @@
 #include <libvlc.h>
 #include <assert.h>
 
+static int vlc_filter_Activate(void *func, bool forced, va_list ap)
+{
+    filter_t *p_filter = va_arg(ap, filter_t *);
+    vlc_filter_open activate = func;
+
+    p_filter->obj.force = forced;
+    int ret = activate(p_filter);
+    if (ret != VLC_SUCCESS)
+        vlc_objres_clear(&p_filter->obj);
+    return ret;
+}
+
+module_t *vlc_filter_LoadModule(filter_t *p_filter, const char *capability,
+                                const char *name, bool strict)
+{
+    const bool b_force_backup = p_filter->obj.force; /* FIXME: remove this */
+    p_filter->p_module = vlc_module_load(p_filter->obj.logger, capability, name, strict,
+                                         vlc_filter_Activate, p_filter);
+    if (p_filter->p_module != NULL) {
+        var_Create(p_filter, "module-name", VLC_VAR_STRING);
+        var_SetString(p_filter, "module-name", module_get_object(p_filter->p_module));
+    }
+
+    p_filter->obj.force = b_force_backup;
+    return p_filter->p_module;
+}
+
 typedef struct chained_filter_t
 {
     /* Public part of the filter structure */