Every 33ms on toplevel windows with a non-dummy, non-offscreen window surface.
Making sure the window surfaces are flushed from their owner thread instead of any other thread that might draw to it and trigger an occasional flush. This removes the need for tracking window surfaces to additionally flush them in win32u.
It will make it easier to implement GL-based window surfaces for compositing, and simplify context management. Instead of one context per window, made current on the flushing thread we can have a single context on each thread owning a window that is used to flush them.
-- v2: win32u: Remove now unnecessary window surfaces list. win32u: Flush window surfaces in process_wine_present. server: Post a WINE_WM_PRESENT for window surface present. win32u: Remove unnecessary window_surface exports.