In cases when cached resource is useable, yet don't have an expiration
timestamp, we provided data to the requester from the cache and the same
data was returned once 304 response was received from the network.
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.
This change enables attaching images to the style with batches and
avoids massive re-allocations. Thus, it improves UI performance
especially at start-up time.
New setPrefetchZoomDelta(optional<uint8_t> delta) method allow overriding
default tile prefetch setting that is defined by the Map instance. The
method can be moved to generic style specification if found to be useful
for gl-js engine.
This will make the `next` buildsystem no longer the `next`, but the
`actual`. The idea is to simplify the build, removing scripts, to
make the platform buildsystem generated by CMake more compatible
with IDEs and make development more streamlined. It will also make
cross compilation easier.
