The last map object to get destroyed will also destroy
the RunLoop (which will also destroy the AsyncTask).
This will clear any reference to the Node default main
loop and let it close.
Downside is that we need to make sure we are always calling
`map.release()`.
It's a premultiplied image. This implies that we were misusing encodePNG
in most cases, as we were passing premultiplied pixels which were then
interpreted as unmultiplied. I changed encodePNG to accept premultipled
pixels, and unpremultiply in the implementations.
The following produces a reference cycle:
var map = new mbgl.Map({
request: function () { ... }
});
map has a reference to the request function, which has a reference to
its closure, which has a reference to map.
Storing a the map options in an internal field, rather than a Persistent,
allows V8 to collect the reference cycle when none of its members have
other references.
