Implementation of multi-sprites in mbgl-core

[geolives-contact]

In order to solve #641.
Implemented in the same way as for Maplibre GL JS.

In detail

In Style JSON, sprite property can now be a string or an array. If this is an array that must contains object(s) with id and urls.
SpriteLoader can now manage multiple sprites in parallel.
Style::Impl now stores the information about how much sprites are loaded. Included a method areSpritesLoaded() to be adequate with the old boolean property that serves the same goal.

When using a string as sprite, that creates internally a Sprite object with id default and the url.
When using an array, there is a parsing to create adequate Sprite objects.

Usage

In images identifiers you need to prepend the sprite id to the image. Example :

"icon-image": "hiking:myimage",

That leads to the image identifier 'myimage' for the sprite object with id 'hiking'.
If id of your sprite is 'default', you don't need to prepend it :

"icon-image": "myimage",

Tip: If your icon-image is property driven, you can use this instead :

"icon-image": [
          "concat",
          "hiking:",
          [
            "get",
            "myproperty"
          ]
        ],

[louwers]

Thank you for your contribution.

Please add tests to style_parser.test.cpp to validate both the old (single string URL) and new (multiple sprites in array) are parsed correctly. Does an empty array get parsed correctly? What is the behavior when specifying the same sprite id twice? Some inspiration can be taken from https://github.com/maplibre/maplibre-gl-js/pull/1805/files#diff-3f25c81a5d56e4b9c90f8e63d1bdc10eeaf8e0de78818ff567d87c5ff5950710

We need to copy the render tests from MapLibre GL JS.

[geolives-contact]

I resolved some of the issues, but one big issue still exist. In the current commit version you can have multiple sprites with the same id. MapLibre GL JS doesn't allow that and I need to handle this case + unit tests.

[geolives-contact]

I implemented checks about id uniqueness.
I added also 2 style parsing tests sprites-missing-fields and sprites-not-same-ids within the style_parser test directory to check attributes of sprites and id uniqueness.
4 additional tests (SpriteAsArrayEmpty,SpriteAsArraySingle, SpriteAsArrayMultiple and SpriteAsString) added to test compatibility with array of sprite or single sprite url inside style_parser.test.cpp.

Seems to be good for me. Tell me if something is wrong.

[louwers]

@Geolives Will do a manual test today!

[louwers]

Sorry for getting back again to this PR so late.

Code looks good, but we still need to add a render test.

[wipfli]

Since this feature already exists in MapLibre GL JS if I remember correctly, can we copy-paste the render test from there?

[louwers]

@wipfli Yes I think so. @Geolives Let me know if you want me to take a look at this, because I have been meaning to do that anyway.

[geolives-contact]

It's probably better if you can do it, as my understandings about the render tests are minor.
Thanks :)

[louwers]

Thanks for your patience with this one. The new render tests passed locally. Thanks for this awesome contribution!

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.2% +34.5Ki  +0.2% +32.0Ki    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-1858-compared-to-main.txt

[louwers]

iOS render test passes locally.

On CI it complains about missing metrics.