Only draw overlays when needed

Some backends don't like empty draw operations, so have an early check.

(cherry picked from commit 0630434292)

vncviewer/DesktopWindow.cxx

 
     fl_clip_box(ox, oy, ow, oh, ox, oy, ow, oh);
 
-    if (offscreen)
-      statsGraph->blend(offscreen, ox - X, oy - Y, ox, oy, ow, oh, 204);
-    else
-      statsGraph->blend(ox - X, oy - Y, ox, oy, ow, oh, 204);
+    if ((ow != 0) && (oh != 0)) {
+      if (offscreen)
+        statsGraph->blend(offscreen, ox - X, oy - Y, ox, oy, ow, oh, 204);
+      else
+        statsGraph->blend(ox - X, oy - Y, ox, oy, ow, oh, 204);
+    }
   }
 
   // Overlay (if active)
 
     fl_clip_box(ox, oy, ow, oh, ox, oy, ow, oh);
 
-    if (offscreen)
-      overlay->blend(offscreen, ox - X, oy - Y, ox, oy, ow, oh, overlayAlpha);
-    else
-      overlay->blend(ox - X, oy - Y, ox, oy, ow, oh, overlayAlpha);
+    if ((ow != 0) && (oh != 0)) {
+      if (offscreen)
+        overlay->blend(offscreen, ox - X, oy - Y, ox, oy, ow, oh, overlayAlpha);
+      else
+        overlay->blend(ox - X, oy - Y, ox, oy, ow, oh, overlayAlpha);
+    }
   }
 
   // Flush offscreen surface to screen