[O6lvl4]

open still opens window 1 and a one-window program needs no change. More windows open with open_window(title, w, h) -> id.

• Per-window calls (size, resized, title, focus, IME, pointer, full screen, background, minimize, attention) are about the window select(id) chose, and gpu render passes draw into its surface. The other windows' surfaces are parked and swapped in by select. present_frame presents every window that drew.
• Every input carries its window (event_window). With more than one window open, a close button is a CLOSE event for that window; with one, pump/wait return false as before. Quitting still makes them return false.
• New windows open hidden, cascade 24 pt from the current one, and show with their first frame. close_window(id) drops a window and its surface.
• set_low_latency and set_option_as_alt apply to every window, including those opened later.

Checked: examples/window draws pixel-identical to main. The new examples/windows opens two windows, draws the scene in each at its own size, and saves both frames.