hw/xwin: A simpleminded attempt at composition

Rather than drawing the window contents from the shadow framebuffer, use
Composite extension redirection to cause the server to maintain a bitmap
image of each top-level X window, and draw the window contents from
that, so that window contents which are occluded in the framebuffer show
correctly in the task bar and task switcher previews.

v2:
Fix incorrect use of memset() found by gcc5

hw/xwin/winshadgdi.c: In function ‘winBltExposedWindowRegionShadowGDI’:
hw/xwin/winshadgdi.c:861:9: warning: ‘memset’ used with constant zero length parameter; this could be due to transposed parameters [-Wmemset-transposed-args]

v3:
Turn on -compositewm by default

v4:
Ignore -swcursor if -compositewm

-swcursor is not compatible with -compositewm (because the window
contents are drawn from an off-screen pixmap, not from the screen
pixmap, where the software cursor will be drawn).

v5:
Update meson.build also
Add -compositewm option to help output
Update CI to install prerequisites

.appveyor.yml

@@ -50,6 +50,7 @@ libXmu-devel,\
 libXpm-devel,\
 libXrender-devel,\
 libXtst-devel,\
+libxcb-composite-devel,\
 libxcb-ewmh-devel,\
 libxcb-icccm-devel,\
 libxcb-image-devel,\