[core] Fix image type of Map::renderStill

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.

platform/node/src/node_map.cpp

@@ -300,7 +300,7 @@ void NodeMap::startRender(std::unique_ptr<NodeMap::RenderOptions> options) {
     map->setBearing(options->bearing);
     map->setPitch(options->pitch);
 
-    map->renderStill([this](const std::exception_ptr eptr, mbgl::UnassociatedImage&& result) {
+    map->renderStill([this](const std::exception_ptr eptr, mbgl::PremultipliedImage&& result) {
         if (eptr) {
             error = std::move(eptr);
             uv_async_send(async);