hw/xwin: improve interaction with native composition
This MR adds options which improve the way multiwindow mode (each top-level X window in it's own Windows window) interacts with native (DWM) composition:
-compositewm: Use Composite extension redirection to maintain a bitmap image of each top-level X window, so that window contents which are occluded when the window is not at the top of the stacking order are shown correctly in task bar and task switcher previews.
-compositealpha: X windows with per-pixel alpha are composited into the native DWM desktop, on Windows version which support this (i.e. a native window can be seen through a semi-transparent X window placed over it)