Generation of HTML is extremely fast, but compressing thousands
of frames for wireframe animated GIFs can take a little while.
Sharing bitmaps between threads required two changes: (1) bitmaps
need to be "frozen" after being drawn; (2) you can't use Path because
BackgroundWorker isn't a STAThread. You can, however, use a
DrawingVisual / DrawingContext to do the rendering. Which is really
what I should have been doing all along; I just didn't know the
approach existed until I was forced to go looking for it.
Also, we now do a "run finalizers" call before generating an animated
GIF. Without it things explode after more than 10K GDI objects have
been allocated.
