If we get a new sprite sheet from the server, we need to merge current
sprite sheet with a new one, while overwiting duplicates and keeping
old unique images in a style.
We used some shared SegmentVectors, e.g. for drawing raster or background tiles.
In longer running maps, this lead to resource accumulation. By storing the SegmentVectors
and the contained gfx::DrawScope objects, we ensure that resources get released
when the associated render objects vanish.
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 change fixes the following problems:
1) Before this change, when multiple symbols were grouped close to the
tile borders, different tiles could place different symbols, because each
tile arbitrary assigned feature ids, and these ids defined the placement
order being applied. This caused artifacts at the tile boundaries.
With this change, in tile mode the placement order of two symbols crossing
a tile border is defined by their anchor `y` values. It means that the
symbols are being placed the same order across all the tiles.
2) Before this change, the engine did not place with priority the symbols
crossing the borders between two neighboring tiles. Now it does, and it
improves a lot symbol placement stability in the tile map mode.
