screen: Make sure we always have a screen

It is possible to configure RANDR so that no outputs are available. This means
awesome would be running with zero screens which leads to all kinds of
funnyness.

Work around this by falling back to other configuration mechanisms if we get no
screens from one of them. After all, screen_scan_x11() will always be able to
add a screen.

Signed-off-by: Uli Schlachter <psychon@znc.in>

screen.c

@@ -75,6 +75,12 @@ screen_add(screen_t new_screen)
     screen_array_append(&globalconf.screens, new_screen);
 }
 
+static bool
+screens_exist(void)
+{
+    return globalconf.screens.len > 0;
+}
+
 static bool
 screen_scan_randr(void)
 {
@@ -150,7 +156,7 @@ screen_scan_randr(void)
 
             p_delete(&screen_res_r);
 
-            return true;
+            return screens_exist();
         }
     }
 
@@ -195,7 +201,7 @@ screen_scan_xinerama(void)
 
         p_delete(&xsq);
 
-        return true;
+        return screens_exist();
     }
 
     return false;