Create and remove layer renderers in renderFrame

Previously, the map renderer would listen for layers being added and
removed from the layers collection, and would create and remove layer
renderers in response to these events.

With this change, layer renderers are only created or removed when
renderFrame is called, which leads to somewhat simpler code.

We still need to listen to changes to the layers collection, but now
these only trigger a new render.

This new approach also has an advantage when layers change order.
Swapping the order of two layers involves removing one and re-inserting
it elsewhere.  With the old approach, this would cause the deletion and
re-creation of the layer renderer.  With this new approach, the layer
renderer is preserved.
Remove layer renderers in post render function
Track changes to layers in the map
Rename wms.js to wmssource.js
Give defaults.{js,exports} unique filenames
Give wgs84.js files unique filenames
Rename test files to match source files
Rename projection files for consistency
Merge pull request #556 from twpayne/consistent-and-unique-filenames

Consistent and unique filenames
Handle obscure edge case where deferred post render function receives a null frame state
ZoomSlider uses non-existing View2D functions
Update Bing Maps key (Expiration Date: 07/16/2013)
Merge pull request #554 from elemoine/bing-maps-key

Bing Maps key has expired
Merge pull request #553 from elemoine/fix-zoomslider

ZoomSlider example is broken
Update Bing Maps key, see #554
Merge pull request #561 from fredj/bingkey

Update Bing Maps key, see #554
Merge pull request #542 from twpayne/simplify-renderer-events

Create and remove layer renderers in renderFrame
Always create layer renderer, even when layer is invisible or not ready

This is necessary because it is the layer renderer that is responsible
for triggering a render when a layer loads or becomes visible.
Render instead of requestRenderFrame when single image loads
Merge pull request #562 from twpayne/renderer-fixes

Renderer fixes