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.
Test whenever destroying a Map object with requests pending
works and is done quickly.
The test injects artificial delays in selected requests and
tries to destroy the Map object afterwards.
Currently glyph requests are skipped because there is a bug
being worked on in a different issue.
