WMTS imagery source is blocked by CORS policy while openlayer displays it

[airnez]

Context

Some raster data (here WMTS) are getting their requests blocked by CORS policy. The same data can be displayed in openlayer without problems.

Steps to Reproduce

1. Install the allow-cors extension in chrome
2. Add this layer to the view_3d_map example :

            var wmtsSource = new itowns.WMTSSource({
                url: 'https://geo.pnrsud.fr/geoserver/gwc/service/wmts',
                crs: 'EPSG:3857',
                name: 'dynmap:scan25_1_paca_2022',
                tileMatrixSet: 'EPSG 3857',
                version: '1.0.0',
                style: 'raster',
                format: 'image/jpeg',
                isInverted: false,
                tileMatrixCallback: (zoomLevel) => 'EPSG 3857:' + zoomLevel
            });

4. zoom to the french city "Alès"
5. See that allowing CORS enables displaying the layer

[jailln]

After analysis it turns out the cors error is legitimate since the server has no cors configuration for images (see https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image#web_server_configuration for an example of server configuration to allow cors on images)