avfilter/formats: Avoid reallocations for video in ff_all_formats()

Up until now, the list of pixfmts is reallocated every time an entry
is added to it; there are currently 196 pixel formats, so this matters:
It causes 5541704 calls to av_realloc_array() in a typical FATE run,
which is the majority for said function (8095768 calls) and even
a large chunk of the calls to av_realloc() itself (12589508 calls).

Fix this by using ff_formats_pixdesc_filter() instead.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/formats.c

@@ -435,11 +435,7 @@ AVFilterFormats *ff_all_formats(enum AVMediaType type)
     AVFilterFormats *ret = NULL;
 
     if (type == AVMEDIA_TYPE_VIDEO) {
-        const AVPixFmtDescriptor *desc = NULL;
-        while ((desc = av_pix_fmt_desc_next(desc))) {
-            if (ff_add_format(&ret, av_pix_fmt_desc_get_id(desc)) < 0)
-                return NULL;
-        }
+        return ff_formats_pixdesc_filter(0, 0);
     } else if (type == AVMEDIA_TYPE_AUDIO) {
         enum AVSampleFormat fmt = 0;
         while (av_get_sample_fmt_name(fmt)) {