[mohansur]

On X11, vkcube flickers while its window is resized.

• Implement _NET_WM_SYNC_REQUEST for both xcb and xlib code paths.
• Release the counter only once the new-size frame has reached the window. This uses VK_KHR_present_wait when the driver supports it.
• otherwise waits for the next present at the new size, though this way is not guaranteed to avoid the flicker, in my testing this looks fine as well.

Tested on gnome-shell & kwin compositor.

Developed with assistance from Claude, i reviewed and tested the changes.

Closes: #1272

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