This should help unifying the interface and get rid of the get_info callback.
Later, I think we could create bitmaps over sections shared with some DWM process and/or host compositor.
-- v2: win32u: Create a HBITMAP backing the window surface pixels. winex11: Create a HBITMAP for the allocated surface pixels. winex11: Create XImage before initializing the window surface. win32u: Pass BITMAPINFO and a HBITMAP to window_surface_init. win32u: Move the window surface color bits to the common struct.