Add support for images in data tiles

[tschaub]

This makes it so data tiles can contain image data in addition to array data.

Adding support for images in data tiles makes sense as part of a larger plan to render all raster tiles with the WebGL tile renderer. The plan could look something like this:

1. Add support for images in data tiles (this branch).
2. Add a new ImageTile (or similarly named) source that extends the DataTile source. This would take a url option that would be a convenience for constructing a loader. The url could be string|Array<string>|function(number, number, number):string (the function form would return a url given z, x, and y tile coordinates. Deprecate the (canvas) tile layer, the TileImage source, and lots of other things.
3. In a major release, rename the WebGLTile layer to Tile and remove all the deprecated things. The DataTile things could be renamed to RasterTile (so RasterTile and VectorTile would be the two tile classes in the API).

[github-actions]

📦 Preview the website for this branch here: https://deploy-preview-14258--ol-site.netlify.app/.

[ahocevar]

I agree on most parts of the plan, except for the deprecation of the Canvas tile layer. The reason is canvas reuse on maps that mix raster tile and vector (tile) layers. For such scenarios, it is better to stay within a renderer type (canvas or WebGL).

An alternative to deprecating the canvas tile layer (renderer) could be to make the future image (data) tiles work with the canvas renderer too.

[tschaub]

Good point about the canvas renderer. I put up #14262 with a work in progress implementation of a new ImageTile source.