Optimize layout and image generation

[springmeyer]

Spritezero has some inefficiencies in the image handling code. This fixes the problems.

Previously:

• Images were encoded to PNG format in generateLayout, only to be then decoded again in generateImage (so the encoding served no purpose). PNG encoding is known to be expensive (since it involves memory allocations and gzip compression).
• Images were encoded to png format in generateLayout even when the underlying image was never needed. When options.format was present, denoting only a data layout, then the only thing needed from the image was the width and height (and not the encoded buffer).

Now:

• Images are no longer encoded. Instead the mapnik.Image instance is passed along to generateImages. That function uses mapnik.blend which supports passing mapnik.Image objects directly since https://github.com/mapnik/node-mapnik/pull/905.
• The code inside generateLayout is simplified to avoid needing an async queue

This closes https://github.com/mapbox/spritezero/issues/7

/cc @mapbox/static-apis @mapbox/map-design-api

[springmeyer]

Thinking about version to bump to for this change... While the public API has not broken, if any code using generateLayout was doing anything unique to the output (beyond just passing it to generateImage) then the mapnik.Image objects instead of encoded PNG buffers may cause issues.

So, I'm thinking it is safest to bump to 7.0.0 for when this is released.

@jseppi should I update the changelog and bump the version in this PR or should I do it in master after?

[jseppi]

should I update the changelog and bump the version in this PR or should I do it in master after?

@springmeyer: Either way is fine with me currently since there aren't any outstanding changes in master