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 callback will be called when any of the resources requested
from the network failed to load.
There is not recovery implemented yet, so the closest thing to
reseting the Map object and reloading all the resources would
be setting a new style.
Both Android and iOS now use pause/resume. The map thread is
always running, and be able to receive messages. When paused,
it will just refuse to render.
