New image tile source

[tschaub]

This adds a new ImageTile source that extends the DataSource as a convenience for working with image tiles. This is part 2 in the plan mentioned in #14258.

The simplest example of its use looks like this:

import Layer from 'ol/layer/WebGLTile.js';
import Map from 'ol/Map.js';
import Source from 'ol/source/ImageTile.js';
import View from 'ol/View.js';

const map = new Map({
  target: 'map',
  layers: [
    new Layer({
      source: new Source({
        url: 'https://tile.openstreetmap.org/{z}/{x}/{y}.png',
      }),
    }),
  ],
  view: new View({
    center: [0, 0],
    zoom: 0,
  }),
});

The source is purely a convenience for creating an image loader given a URL. The url option can be a single URL template, an array of templates, or a function that returns a URL. The resulting loader returns a promise that creates an image, sets its src, and resolves when the image loads. For more complex image loading scenarios, the source can be passed a loader option.

When a data tile is disposed, we now dispatch an abort event on an abort controller's signal. The GeoTIFF source shows how the abort signal can be passed to fetch.

There is probably more to do to support all the functionality of the current image tile stuff, but I think this makes for a simpler foundation than what we have currently. If we continue with this, we could eventually remove things like:

• ol/source/UrlTile.js
• ol/source/TileImage.js
• ol/source/XYZ.js
• ol/ImageTile.js
• ol/tileurlfunction.js

Still todo:

• deal with interim tiles

[github-actions]

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[ahocevar]

Thanks for your work on this, @tschaub. I'll be glad to review this, but please be patient. In the process of reviewing, I'd like to play around with the new ImageTile source, with the goal of making it compatible with the canvas tile renderer (or vice verse).

[tschaub]

No rush, @ahocevar. I would also like to do the same. One significant difference is that the data tile source doesn’t cache tiles. So this will mean adding a cache to the canvas renderer.

[tschaub]

I've squashed these changes in order to make it easier to deal with the conflicts. New pull request #15568.