Treat icon-size as scale

[KaiVolland]

This changes the interpretation of the icon-size property to match the description of the mapbox style definition: https://docs.mapbox.com/style-spec/reference/layers/#layout-symbol-icon-size

If an icon-size is specified it will get multiplied with the average of width and height to get the real size in pixels.

[dnlkoch]

:tada: This PR is included in version 5.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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