avfilter: Deduplicate default audio inputs/outputs

Lots of audio filters use very simple inputs or outputs:
An array with a single AVFilterPad whose name is "default"
and whose type is AVMEDIA_TYPE_AUDIO; everything else is unset.

Given that we never use pointer equality for inputs or outputs*,
we can simply use a single AVFilterPad instead of dozens; this
even saves .data.rel.ro (4784B here) as well as relocations.

*: In fact, several filters (like the filters in af_biquads.c)
already use the same inputs; furthermore, ff_filter_alloc()
duplicates the input and output pads so that we do not even
work with the pads directly.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/f_loop.c

@@ -300,13 +300,6 @@ static const AVFilterPad ainputs[] = {
     },
 };
 
-static const AVFilterPad aoutputs[] = {
-    {
-        .name          = "default",
-        .type          = AVMEDIA_TYPE_AUDIO,
-    },
-};
-
 const AVFilter ff_af_aloop = {
     .name          = "aloop",
     .description   = NULL_IF_CONFIG_SMALL("Loop audio samples."),
@@ -315,7 +308,7 @@ const AVFilter ff_af_aloop = {
     .activate      = aactivate,
     .uninit        = auninit,
     FILTER_INPUTS(ainputs),
-    FILTER_OUTPUTS(aoutputs),
+    FILTER_OUTPUTS(ff_audio_default_filterpad),
 };
 #endif /* CONFIG_ALOOP_FILTER */