monitor: Add scale 1 if no other supported scale was added

We currently have a hard coded limit on logical monitor sizes, meant
for filtering out monitor scales that would result in awkward desktop
sizes. This has the side effect of also disqualifying scale 1 for
resolutions that themself are lower than the mentioned limit. To avoid
listing no supported scales, always add the fallback scale 1 if no
other was added.

https://bugzilla.gnome.org/show_bug.cgi?id=787477

src/backends/meta-monitor.c

@@ -1550,6 +1550,14 @@ meta_monitor_calculate_supported_scales (MetaMonitor                *monitor,
         }
     }
 
+  if (supported_scales->len == 0)
+    {
+      float fallback_scale;
+
+      fallback_scale = 1.0;
+      g_array_append_val (supported_scales, fallback_scale);
+    }
+
   *n_supported_scales = supported_scales->len;
   return (float *) g_array_free (supported_scales, FALSE);
 }