Add visualization sets to exported HTML

We now generate GIF images for visualizations and add inline
references to them in the HTML output.

Images are scaled using the HTML img properties.  This works well
on some browsers, but others insist on "smooth" scaling that blurs
out the pixels.  This may require a workaround.

An extra blank line is now added above visualizations.  This helps
keep the image and data visually grouped.

The Apple II bitmap test project was updated to have a visualization
set with multiple images at the top of the file.

SourceGen/Visualization.cs

@@ -47,7 +47,8 @@ namespace SourceGen {
         public ReadOnlyDictionary<string, object> VisGenParams { get; private set; }
 
         /// <summary>
-        /// Cached reference to 2D image, useful for thumbnails.  Not serialized.
+        /// Cached reference to 2D image, useful for thumbnails.  Not serialized.  This always
+        /// has an image reference; in times of trouble it will point at BROKEN_IMAGE.
         /// </summary>
         /// <remarks>
         /// Because the underlying data never changes, we only need to regenerate the