fix(windows): rerender while windows exist

The changes to window animator mean we don't need to track if any
windows were updated, just checking if windows exist is sufficient to
keep rendering.

See #154

lua/notify/windows/init.lua

@@ -30,15 +30,13 @@ end
 function WindowAnimator:render(queue, time)
   self:push_pending(queue)
   if vim.tbl_isempty(self.win_stages) then
-    return nil
+    return false
   end
-  local updated = false
   local open_windows = vim.tbl_keys(self.win_stages)
   for win, _ in pairs(self.win_stages) do
-    local res = self:_update_window(time, win, open_windows)
-    updated = updated or res
+    self:_update_window(time, win, open_windows)
   end
-  return updated
+  return true
 end
 
 function WindowAnimator:push_pending(queue)
@@ -152,7 +150,6 @@ function WindowAnimator:_update_window(time, win, open_windows)
 
   if not win_goals then
     self:_remove_win(win)
-    return true
   end
 
   -- If we don't animate, then we move to all goals instantly.
@@ -177,13 +174,11 @@ function WindowAnimator:_update_window(time, win, open_windows)
     self:_start_timer(win)
   end
 
-  local updated = self:_advance_win_state(win, win_goals, time)
+  self:_advance_win_state(win, win_goals, time)
 
   if self:_is_complete(win, win_goals) and not win_goals.time then
     self:_advance_win_stage(win)
   end
-
-  return updated
 end
 
 function WindowAnimator:_is_complete(win, goals)