Add border_size_dirty flag to avoid unnecessary shape queries

[jEsuSdA]

Summary

Adds a per-window border_size_dirty flag so border_size() is only called for windows whose shape has actually changed, rather than for all windows whenever clip_changed is true.

Problem

border_size() queries the X server for the window shape (via XFixesCreateRegionFromWindow or XShapeQueryExtents). This is expensive. Currently, when clip_changed is true (which happens on map, unmap, restack, and configure events), all windows have their border_size destroyed and recreated, even if their shape hasn't changed.

Solution

Add a Bool border_size_dirty field to the win struct. The flag is set:

• in map_win() when a window becomes viewable
• in do_configure_win() when configure_size_changed is true

In paint_all(), the window's border_size is only destroyed and rebuilt when clip_changed is true or when border_size_dirty is set for that specific window. The flag is cleared after rebuilding.

Scope

• cm-window.h — adds one Bool field to the win struct
• fastcompmgr.c — sets the flag in map_win() and do_configure_win(), checks it in paint_all()
• make clean && make produces zero warnings, zero errors

Impact

Reduces the number of expensive X server shape queries, especially when many windows are visible and only one window changes (e.g. a new window is mapped or a single window is resized). The exact reduction depends on the number of visible windows and the frequency of clip changes.

Also includes a !w->destroyed guard before calling border_size(), preventing a potential shape query on a window that has already been destroyed.