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/vf_stack.c

@@ -402,7 +402,6 @@ static const AVFilterPad outputs[] = {
         .type          = AVMEDIA_TYPE_VIDEO,
         .config_props  = config_output,
     },
-    { NULL }
 };
 
 #if CONFIG_HSTACK_FILTER
@@ -416,7 +415,7 @@ const AVFilter ff_vf_hstack = {
     .priv_size     = sizeof(StackContext),
     .priv_class    = &hstack_class,
     .query_formats = query_formats,
-    .outputs       = outputs,
+    FILTER_OUTPUTS(outputs),
     .init          = init,
     .uninit        = uninit,
     .activate      = activate,
@@ -436,7 +435,7 @@ const AVFilter ff_vf_vstack = {
     .priv_size     = sizeof(StackContext),
     .priv_class    = &vstack_class,
     .query_formats = query_formats,
-    .outputs       = outputs,
+    FILTER_OUTPUTS(outputs),
     .init          = init,
     .uninit        = uninit,
     .activate      = activate,
@@ -463,7 +462,7 @@ const AVFilter ff_vf_xstack = {
     .priv_size     = sizeof(StackContext),
     .priv_class    = &xstack_class,
     .query_formats = query_formats,
-    .outputs       = outputs,
+    FILTER_OUTPUTS(outputs),
     .init          = init,
     .uninit        = uninit,
     .activate      = activate,