Don't save separate wavs for each selected line

Instead, save a single wav spanning the full range of all selected
lines.

aegisub/src/command/audio.cpp

@@ -220,11 +220,17 @@ struct audio_save_clip : public Command {
 
 	void operator()(agi::Context *c) {
 		Selection sel = c->selectionController->GetSelectedSet();
+		if (sel.empty()) return;
+
+		AssTime start = INT_MAX, end = 0;
 		for (Selection::iterator it = sel.begin(); it != sel.end(); ++it) {
+			start = std::min(start, (*it)->Start);
+			end = std::max(end, (*it)->End);
+		}
+
 		c->audioController->SaveClip(
 			wxFileSelector(_("Save audio clip"), "", "", "wav", "", wxFD_SAVE|wxFD_OVERWRITE_PROMPT, c->parent),
-				TimeRange((*it)->Start, (*it)->End));
-		}
+			TimeRange(start, end));
 	}
 };