[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.

test/api/api_misuse.cpp

@@ -45,7 +45,7 @@ TEST(API, RenderWithoutStyle) {
     Map map(view, fileSource, MapMode::Still);
 
     std::promise<std::exception_ptr> promise;
-    map.renderStill([&promise](std::exception_ptr error, UnassociatedImage&&) {
+    map.renderStill([&promise](std::exception_ptr error, PremultipliedImage&&) {
         promise.set_value(error);
     });