Cache find_client_win results to avoid repeated XQueryTree scans

[jEsuSdA]

Summary

Caches find_client_win() results to eliminate repeated expensive XQueryTree() recursive scans on every frame.

Problem

find_client_win() recursively calls XQueryTree() to locate the actual client window within a toplevel window hierarchy. This is expensive and currently called:

• on every call to get_frame_extents() (which reads _NET_FRAME_EXTENTS)
• in win_paint_needed() when the hidden state is still HIDDEN_UNKNOWN

For windows without a client window (e.g. override_redirect windows, or windows where the WM doesn't create a client), this results in a full recursive scan of the window hierarchy on every frame.

Solution

Add Bool client_id_resolved and Window client_id fields to the win struct. The first call to find_client_win() stores the result; subsequent calls reuse the cached value.

The cache is invalidated on ReparentNotify events because the window hierarchy may have changed (e.g., when the window manager restarts and reparents windows).

Changes

• cm-window.h — adds client_id_resolved and client_id fields to win struct
• fastcompmgr.c — uses the cache in get_frame_extents() and win_paint_needed(), invalidates on reparent in add_damage_if_hidden_changed()

Scope

• cm-window.h and fastcompmgr.c only
• make clean && make produces zero warnings, zero errors
• Fields are zero-initialized by calloc() in add_win()

Impact

Eliminates repeated XQueryTree() recursive scans for windows whose client window doesn't change. Most noticeable for windows without a client (e.g. override_redirect popups, menus), which previously triggered a full hierarchy scan on every frame.