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.

Since it may have noticeable performance impact, add an option to
disable it.

Fixes QubesOS/qubes-issues#4351

Author: marmarek

gui-agent/vmside.c

@@ -86,6 +86,7 @@ struct _global_handles {
     unsigned int clipboard_data_len;
     int log_level;
     int sync_all_modifiers;
+    int composite_redirect_automatic;
 };
 
 struct window_data {
@@ -1943,11 +1944,12 @@ void handle_guid_disconnect()
 
 void usage()
 {
-    fprintf(stderr, "Usage: qubes_gui [-v] [-q] [-h]\n");
+    fprintf(stderr, "Usage: qubes_gui [options]\n");
     fprintf(stderr, "       -v  increase log verbosity\n");
     fprintf(stderr, "       -q  decrease log verbosity\n");
     fprintf(stderr, "       -m  sync all modifiers before key event (default)\n");
     fprintf(stderr, "       -M  sync only Caps Lock key event\n");
+    fprintf(stderr, "       -c  turn off composite \"redirect automatic\" mode\n");
     fprintf(stderr, "       -h  print this message\n");
     fprintf(stderr, "\n");
     fprintf(stderr, "Log levels:\n");
@@ -1963,7 +1965,8 @@ void parse_args(Ghandles * g, int argc, char **argv)
     // defaults
     g->log_level = 0;
     g->sync_all_modifiers = 1;
-    while ((opt = getopt(argc, argv, "qvhmM")) != -1) {
+    g->composite_redirect_automatic = 1;
+    while ((opt = getopt(argc, argv, "qvchmM")) != -1) {
         switch (opt) {
             case 'q':
                 g->log_level--;
@@ -1977,6 +1980,9 @@ void parse_args(Ghandles * g, int argc, char **argv)
             case 'M':
                 g->sync_all_modifiers = 0;
                 break;
+            case 'c':
+                g->composite_redirect_automatic = 0;
+                break;
             case 'h':
                 usage();
                 exit(0);
@@ -2013,11 +2019,19 @@ 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);
+            (g.composite_redirect_automatic ?
+              CompositeRedirectAutomatic :
+              CompositeRedirectManual));
     for (i = 0; i < ScreenCount(g.display); i++)
         XSelectInput(g.display, RootWindow(g.display, i),
                 SubstructureNotifyMask);