default to loading a blank png when a tile request fails

[jgravois]

resolves #759

it seems sensible to me that we just load a transparent .png by default when individual tile requests return a 404 error and easiest to just grab it directly from the Esri knowledge base article i tracked down.

Article ID: 36939
http://support.esri.com/en/knowledgebase/techarticles/detail/36939

[blq]

I notice that this blank tile png is hosted on a plain http url.
Would it be possible for you to host it on https instead? A site that uses https for maps will otherwise block it and get lots of mixed content warnings!

[jgravois]

Would it be possible for you to host it on https instead?

thats a good idea. the current resource doesn't support encrypted requests so i'll have to think a bit about where else to store the .png.

[blq]

OK. or use an inline base64 image?

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAQAAAAEABAMAAACuXLVVAAAAA1BMVEUzNDVszlHHAAAAAXRSTlMAQObYZgAAAAlwSFlzAAAAAAAAAAAB6mUWpAAAADZJREFUeJztwQEBAAAAgiD/r25IQAEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA7waBAAABw08RwAAAAABJRU5ErkJggg==

(the above is exact blank256.png encoded, but I haven't tested live)

[patrickarlt]

@jgravois what @blq would suffice pretty nicely without having to host an additional resource.