avfilter/avfilter: Add numbers of (in|out)pads directly to AVFilter

Up until now, an AVFilter's lists of input and output AVFilterPads
were terminated by a sentinel and the only way to get the length
of these lists was by using avfilter_pad_count(). This has two
drawbacks: first, sizeof(AVFilterPad) is not negligible
(i.e. 64B on 64bit systems); second, getting the size involves
a function call instead of just reading the data.

This commit therefore changes this. The sentinels are removed and new
private fields nb_inputs and nb_outputs are added to AVFilter that
contain the number of elements of the respective AVFilterPad array.

Given that AVFilter.(in|out)puts are the only arrays of zero-terminated
AVFilterPads an API user has access to (AVFilterContext.(in|out)put_pads
are not zero-terminated and they already have a size field) the argument
to avfilter_pad_count() is always one of these lists, so it just has to
find the filter the list belongs to and read said number. This is slower
than before, but a replacement function that just reads the internal numbers
that users are expected to switch to will be added soon; and furthermore,
avfilter_pad_count() is probably never called in hot loops anyway.

This saves about 49KiB from the binary; notice that these sentinels are
not in .bss despite being zeroed: they are in .data.rel.ro due to the
non-sentinels.

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

libavfilter/af_channelmap.c

@@ -387,7 +387,6 @@ static const AVFilterPad avfilter_af_channelmap_inputs[] = {
         .filter_frame   = channelmap_filter_frame,
         .config_props   = channelmap_config_input,
     },
-    { NULL }
 };
 
 static const AVFilterPad avfilter_af_channelmap_outputs[] = {
@@ -395,7 +394,6 @@ static const AVFilterPad avfilter_af_channelmap_outputs[] = {
         .name = "default",
         .type = AVMEDIA_TYPE_AUDIO
     },
-    { NULL }
 };
 
 const AVFilter ff_af_channelmap = {
@@ -405,6 +403,6 @@ const AVFilter ff_af_channelmap = {
     .query_formats = channelmap_query_formats,
     .priv_size     = sizeof(ChannelMapContext),
     .priv_class    = &channelmap_class,
-    .inputs        = avfilter_af_channelmap_inputs,
-    .outputs       = avfilter_af_channelmap_outputs,
+    FILTER_INPUTS(avfilter_af_channelmap_inputs),
+    FILTER_OUTPUTS(avfilter_af_channelmap_outputs),
 };