Add Atari 2600 sprite/playfield visualizer

First swing at a visualizer for Atari 2600 sprites and playfields.
Won't necessarily present an accurate view of what is displayed on
screen, but should provide a reasonable shape for data stored in
the obvious way.

The Adventure playfields looked squashed, so I added a simple row
duplication value.

Also, minor improvements to visualizers generally:
- Throw an exception, rather than an Assert, in VisBitmap8 when the
  arguments are bad.
- Show the exception in the Visualization Edit dialog.
- If generation fails and we don't have an error message, show a
  generic "stuff be broke" string.
- Set focus on OK button in Visualization Set Edit after editing,
  so you can hit Enter twice after renaming a tag.

SourceGen/RuntimeData/Help/visualization.html

@@ -108,7 +108,7 @@ Some less-common parameters include:</p>
     visualizer will default to no interleave (stride == 1).</li>
 </ul>
 
-<h3>Apple II - VisHiRes</h3>
+<h3>Apple II - Apple/VisHiRes</h3>
 
 <p>There is no standard format for small hi-res bitmaps, but certain
 arrangements are common.  The script defines three generators:</p>
@@ -139,6 +139,23 @@ but has no effect on black or white.</p>
 <p>The converter generates one output pixel for every source pixel, so
 half-pixel shifts are not rendered.</p>
 
+<h3>Atari 2600 - Atari/VisAtari2600</h3>
+
+<p>The Atari 2600 graphics system has registers that determine the
+appearance of a sprite or playfield on a single row.  The visualization
+generator works for data stored in a straightforward fashion.</p>
+
+<ul>
+  <li><b>Sprite</b> - basic 1xN sprite, converted to an image 8 pixels
+    wide.</li>
+  <li><b>Playfield</b> - assumes PF0,PF1,PF2 are stored in that order,
+    multiple entries following each other.  Specify the number of
+    3-byte entries as the height.
+    Since most playfields aren't the full height of the screen,
+    it will tend to look squashed.  Use the "row duplication" feature
+    to repeat each row N times to make it look more like it should.</li>
+</ul>
+
 </div>
 
 <div id="footer">