Allow querying background opacity via XTGETTCAP

kittens/query_terminal/main.py

@@ -202,6 +202,20 @@ class Background(Query):
         return (w.screen.color_profile.default_bg or get_options().background).as_sharp
 
 
+@query
+class BackgroundOpacity(Query):
+    name: str = 'background_opacity'
+    help_text: str = 'The current background opacity as a number between 0 and 1'
+
+    @staticmethod
+    def get_result(opts: Options, window_id: int, os_window_id: int) -> str:
+        from kitty.fast_data_types import background_opacity_of
+        ans = background_opacity_of(os_window_id)
+        if ans is None:
+            ans = 1.0
+        return f'{ans:g}'
+
+
 @query
 class ClipboardControl(Query):
     name: str = 'clipboard_control'