Fix the export map example

As @barbalex discovered, the map needs to be rendered synchronously for the export map example to work.

While it is nice to have a justification for synchronous map rendering, I'll point out that it really shouldn't be necessary to re-render the map in this example.  Ideally, a click would just grab the pixels that are already rendered.  So I don't think that this is a solid justification for synchronous map rendering.  But this would involve exposing the map canvas in a different way (not just in a render event).  If we didn't have a `renderSync` method and didn't want to expose another method for getting the map canvas, we still could make this example work by rendering asynchronously, creating a new (hidden) anchor element on the first `postcompose`, setting its `href` to the rendered data, and invoking its `click` method.  But this would make for some pretty ugly example code.

Anyway, nice fix @barbalex (can you tell there is some history here?).

Fixes #2341.