Modernize the Shadings structure, in src/core/pattern.js, to use …

…standard classes

This patch replaces the old structure with a abstract base-class, which the new RadialAxial/Mesh-shading classes then inherit from.[1]
The old `MeshClosure` can now be removed, since it's not necessary any more, and most of the functions inside of it are now instead methods on the new `MeshShading` class. This is particularily nice, in my opinion, since we previously were *manually* passing around a reference to the current `Mesh`-instance.

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[1] If we want/need to, in the future, split e.g. the Mesh-handling into multiple classes that should now be easy to do.