capture: refuse the multi-track ranged item render, silence the track's children and receives for it, and gate every exact-bounds capture on its frame count

src/core/capture/render_settings.cpp

@@ -112,6 +112,13 @@ SourceMode sourceModeForScope(CaptureScope scope, bool itemExtentIsWindow) {
     return SourceMode::SelectedTracks;
 }
 
+bool isMultiTrackRangedItemRender(CaptureScope scope, SourceMode mode,
+                                  int sourceTrackCount) {
+    return scope == CaptureScope::Item
+        && mode  == SourceMode::SelectedTracks
+        && sourceTrackCount > 1;
+}
+
 RangeSource inferRangeSource(bool hasRazorArea) {
     // Razor wins when present; otherwise the time selection. Orthogonal to scope.
     return hasRazorArea ? RangeSource::Razor : RangeSource::TimeSelection;