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.
