gui-agent: switch to automatic composition mode, fix in-vm screenshots

For in-vm screenshots (including screen sharing etc) to work, window
composition buffers needs to be rendered back to root window. Using
"automatic" composition mode does that.

Fixes QubesOS/qubes-issues#4351

Author: marmarek

gui-agent/vmside.c

@@ -2013,11 +2013,18 @@ int main(int argc, char **argv)
     mkghandles(&g);
     ghandles_for_vchan_reinitialize = &g;
     parse_args(&g, argc, argv);
+    /* Turn on Composite for all children of root window. This way X server
+     * keeps separate buffers for each (root child) window.
+     * There are two modes:
+     *  - manual - this way only off-screen buffers are maintained
+     *  - automatic - in addition to manual, widows are rendered back to the
+     *  root window
+     */
     for (i = 0; i < ScreenCount(g.display); i++)
         XCompositeRedirectSubwindows(g.display,
                 RootWindow(g.display, i),
-                //                                           CompositeRedirectAutomatic);
-            CompositeRedirectManual);
+            CompositeRedirectAutomatic);
+//            CompositeRedirectManual);
     for (i = 0; i < ScreenCount(g.display); i++)
         XSelectInput(g.display, RootWindow(g.display, i),
                 SubstructureNotifyMask);