Add support for multi-sprites in styles

[acalcutt]

maplibre-gl-native node-v5.4.0 adds support for multi-sprites ( https://maplibre.org/maplibre-style-spec/sprite/#multiple-sprite-sources ).

This PR allows tileserver-gl to handle an array of local sprites, in a format like

  "sprite": [
    {
      "id": "colored",
      "url": "{styleJsonFolder}/colored_sprites"
    },
    {
      "id": "default",
      "url": "{styleJsonFolder}/sprite"
    }
  ],

you can also specify urls, like shown in the style spec

  "sprite": [
    {
      "id": "colored",
      "url": "https://tiles.wifidb.net/styles/WDB_GPSTRAILS/sprite/colored_sprites"
    },
    {
      "id": "default",
      "url": "https://tiles.wifidb.net/styles/WDB_GPSTRAILS/sprite"
    }
  ],

Example
I updated this ATV trail map to use two different sets of sprites, 'default' and 'colored'. You can see the default icons and colored icons show as expected
https://tiles.wifidb.net/styles/WDB_GPSTRAILS/?raster#15/45.0491/-69.8739

[mnutt]

If you wanted to you could do

const { name = 'sprite', scale = '', format, id } = req.params;

[mnutt]

I don't have an issue with how it is here, but I wonder if it might be worth having a helper method for doing this replacement?

Another option would be to start by normalizing sprites when initially loading styles?

if (styleJSON.sprite) {
  if (!Array.isArray(styleJSON.sprite)) {
    styleJSON.sprite = [{id: 'default', url: styleJSON.sprite }]; 
  }

  for (let spriteItem of styleJSON.sprite) {
    if (!httpTester.test(spriteItem.url)) {
      spriteItem.url = ...
    }
  }
}

[acalcutt]

I do agree this looks better and is simpler. My only concern my previous way preserved the existing sprite format, so if you were not using multi-sprites it would function as before and work with older versions of mapbox/maplibre that do not support multi-sprite.

Where this way basically forces multi-sprites on the users style whether they are using them or not. So it would only work with newer maplibre-js that supports that.

[acalcutt]

I'm on the fence about reverting this change. I don't know if I want to break clients that doesn't support multi-sprite if the person making the style hasn't chosen to use them. it does look a lot nicer though.

[mnutt]

super minor thing, you can move the || '' in to make it the default:

Suggested change

	const scale = allowedSpriteScales(req.params.scale) || '';
	const scale = allowedSpriteScales(req.params.scale, { defaultValue: '' });

[acalcutt]

when I try this, scale ends up 'undefined' if no scale is set, so it ends up putting that into the file name. I see this at the console

Sprite load error: C:\Users\andrew.EIRI\Desktop\test_data\styles\klokantech-basic\spriteundefined.png