Add an option to turn on the use of the X window's alpha channel in
multiwindow mode, i.e. this uses the X window's alpha channel for
compositing into the native desktop.

This works on W7/Vista (using DwmEnableBlurBehindWindow()), and Windows
10 (using the undocumented SetWindowCompositionAttribute()), but not on
Windows 8/8.1

-compositewm must be enabled for this to be useful, as we only have a
pixmap with an alpha channel for the X window in that case.  The
framebuffer/root window doesn't have one (unless perhaps you are using
the rootless extension, maybe...).

v2:
Update meson.build

Future work:

A window property to control use of alpha?
Option to turn off blur on W7/Vista
Implement _NET_WM_WINDOW_OPACITY