An idea for simplifying and solidifying the graphic features API. All graphic features should be flat and have no user-changeable or user accessible attributes, except for .value for features that subclass BufferManager.

Reasoning: currently VertexCmap allows being set like this:

line.cmap = "viridis"`
line.cmap.transform = array_1d
line.cmap.alpha = 0.5

I think this is confusing since it is inconsistent compared to all other graphic features, and it can also make events confusing. For example, what if you want an event for the transform or alpha but no the cmap?

Specific to cmaps for PositionsGraphics we can have an independent cmap_transform feature that only manages the cmap transform and doesn't touch the cmap or alpha. If an alpha is set on the graphic then it will read the alpha value set on the graphic to set that dimension in the final RGBA array to set the colors buffer. Similarly, if a user decides to set VertexColors it will read the alpha feature of that graphic. Perhaps we can allow alpha to be set to None or a float between [0.0, 1.0], can decide this detail later.

Anyways, basic idea is that from our experience I think we can make a formal structure that all GraphicFeatures should follow. Pretty much all of them already follow this except for VertexCmap:

1. GraphicFeatures should not have any user-end attributes or methods. Except for BufferManagers which have .value.
2. They must only have one type of event which corresponds to their single settable attribute value (ex. cmap, cmap_transform, alpha, hickness, etc.) and nothing else.
3. A GraphicFeature is allowed to access the other features of a graphic when necessary, for example VertexCmap already does this since it's basically a wrapper for VertexColors. But we can go further with this by for example adding VertexAlpha to set the color alpha of a graphic. I think this makes it really easy to set things on the user-end in an independent way, and to also get precise events.

Implemeting this is trivial, I just have to split up VertexCmap. And we can have this is dev docs somewhere that this is the structure of graphic features.