[core] Fix excessive onSpriteLoaded() notifications

What happened:

The excessive `onSpriteLoaded()` notifications were caused by the trick inside `OnlineFileRequest::completed()` implemenation,
which causes receving of two file source updates pointing to the *same* data in case the network response is `notModified` and
the local storage has the sprite sheet already stored inside.

In the SpriteLoader` implementation, it resulted in 2 extra `parseSprite()` calls and 2 extra `onSpriteLoaded()` notifications.

Then, the excessive `onSpriteLoaded()` notifications affected style images and all the unnecessarily added image dupes (btw,
the amount of these dupes `= 2 * number of style images`) were treated by the `RenderOrchestrator` as *updated* images.

Further, `RenderOrchestrator` called `ImageManager::updateImage()` for the updated images and the `ImageManager` populated the
`updatedImageVersions` map.

Finally, at every frame all the geometry tiles iterated through the `updatedImageVersions` to check whether their uploaded
textures needed patching.

All the mentioned above had a negative performance impact.

The fix:

Now, `SpriteLoader` sends `onSpriteLoaded()` notifications only if the data in the file source response is different comparing
to the existing one.

src/mbgl/sprite/sprite_loader.cpp

@@ -58,9 +58,9 @@ void SpriteLoader::load(const std::string& url, FileSource& fileSource) {
         } else if (res.noContent) {
             loader->json = std::make_shared<std::string>();
             emitSpriteLoadedIfComplete();
-        } else {
+        } else if (loader->json != res.data) { // They can be equal, see OnlineFileRequest::completed().
             // Only trigger a sprite loaded event we got new data.
-            loader->json = res.data;
+            loader->json = std::move(res.data);
             emitSpriteLoadedIfComplete();
         }
     });
@@ -73,8 +73,8 @@ void SpriteLoader::load(const std::string& url, FileSource& fileSource) {
         } else if (res.noContent) {
             loader->image = std::make_shared<std::string>();
             emitSpriteLoadedIfComplete();
-        } else {
-            loader->image = res.data;
+        } else if (loader->image != res.data) { // They can be equal - see OnlineFileRequest::completed().
+            loader->image = std::move(res.data);
             emitSpriteLoadedIfComplete();
         }
     });