topcat: more responsive large/slow plots

When navigating (pan/zoom) round plots that take more than a significant
fraction of a second to draw, a plot of an evenly spaced subsample is
prepared during the navigation, and displayed at intermediate positions.
This can give a somewhat jumpy appearance to the plot during navigation
(the number of points changes according to whether you're within a
navigation sequence or at rest), but it gives you better visual feedback
overall, since you see frames more often.

topcat/src/docs/sun253.xml

@@ -18251,6 +18251,8 @@ introduced since the last version:
                 visible histogram is currently in cumulative mode.</li>
             </ul></li>
         <li>Layer plot windows now show progress bar as data is loaded.</li>
+        <li>Layer plot windows more responsive (subsampled intermediate
+            plots displayed during navigation round very large/slow plots).</li>
         </ul>
         </p></dd>
 </dl>