x11: free old root pixmap if we re-create it

we re-create this pixmap in _get_root_pixmap_and_surface() if its width and
height do not match (here we cannot resize unfortunately; if it's smaller
we could allow clipping, but we don't want to do that because of our
various fill modes, and if it's larger, well, the old pixmap is not big
enough and there are no x11 resize calls).

however, we did not free the old pixmap, resulting in a leak every time we
painted the screen.

Signed-off-by: Tycho Andersen <tycho@tycho.pizza>

libqtile/backend/x11/xcbq.py

@@ -630,6 +630,7 @@ def __init__(self, display):
         self.atoms = AtomCache(self)
         self.width = -1
         self.height = -1
+        self.root_pixmap_id = None
 
     def _get_root_pixmap_and_surface(self, screen):
         # Querying the screen dimensions via the xcffib connection does not
@@ -702,6 +703,11 @@ def _update_root_pixmap(self, root_pixmap):
         self.conn.core.ClearArea(0, self.default_screen.root.wid, 0, 0, self.width, self.height)
         self.conn.flush()
 
+        # now that we have drawn the new pixmap, free the old one
+        if self.root_pixmap_id is not None and self.root_pixmap_id != root_pixmap:
+            self.conn.core.FreePixmap(self.root_pixmap_id)
+            self.root_pixmap_id = root_pixmap
+
     def fill(self, screen, background):
         root_pixmap, surface = self._get_root_pixmap_and_surface(screen)