Method `U.loadImage` is incompatible with new MapLibreGL versions

[leopiccionia]

maplibre-gl-js version 4.0.0 removed the callback param from some methods, in favor of returning promises (see changelog).

Since then, method U.loadImage provided by map-gl-utils is not compatible with maplibre-gl-js anymore.

Are you interested in fixing this issue? I can submit a PR, if you don't mind.

Thanks for the great job!

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P.S. Are you interested in expanding the compatibility layer between mapbox-gl-js and maplibre-gl-js? I know at least getClusterExpansionZoom, getClusterChildren, and getClusterLeaves suffer from this kind of incompatibility, but perhaps there are others. If so, I can open a separate issue and PR for them.

[stevage]

PRs for both of those would be amazing. In general I'd really like it to work with both libraries as easily and seamlessly as possible.