avfilter/vf_tpad: fix check for drawing initialization

The check if drawing needs to be initialized and supported formats
should be drawable ones was flawed, as pad_stop/pad_start is only
populated from stop_duration/start_duration after these checks.

To fix that, check the _duration variants as well and for better
readability and maintainability break the check out into its own
helper.

Fixes a regression from 86b252ea9d
Fix #10621

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 6667741029)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_tpad.c

@@ -70,11 +70,17 @@ static const AVOption tpad_options[] = {
 
 AVFILTER_DEFINE_CLASS(tpad);
 
+static int needs_drawing(const TPadContext *s) {
+    return (
+        (s->stop_mode  == MODE_ADD && (s->pad_stop  != 0 || s->stop_duration  != 0)) ||
+        (s->start_mode == MODE_ADD && (s->pad_start != 0 || s->start_duration != 0))
+    );
+}
+
 static int query_formats(AVFilterContext *ctx)
 {
     TPadContext *s = ctx->priv;
-    if ((s->stop_mode == MODE_ADD && s->pad_stop != 0) ||
-        (s->start_mode == MODE_ADD && s->pad_start != 0))
+    if (needs_drawing(s))
         return ff_set_common_formats(ctx, ff_draw_supported_pixel_formats(0));
 
     return ff_set_common_formats(ctx, ff_all_formats(AVMEDIA_TYPE_VIDEO));
@@ -196,8 +202,7 @@ static int config_input(AVFilterLink *inlink)
     AVFilterContext *ctx = inlink->dst;
     TPadContext *s = ctx->priv;
 
-    if ((s->stop_mode == MODE_ADD && s->pad_stop != 0) ||
-        (s->start_mode == MODE_ADD && s->pad_start != 0)) {
+    if (needs_drawing(s)) {
         ff_draw_init(&s->draw, inlink->format, 0);
         ff_draw_color(&s->draw, &s->color, s->rgba_color);
     }