attribute based styles

[ansis]

Currently all features in a bucket are rendered with the same style. It would be awesome to be able to optionally read these these style properties from attributes instead of uniforms. This would let us specify style properties individually for each feature.

We would need to:

• generate necessary shader code on initial load (switch between uniform/attribute)
• create more flexible buffer views (not just a small number of predefined ones)
• be able to match buffer views to shaders, automatically binding attributes

This may not be that important because for many use cases we could just split data into multiple buckets. However, I think the flexibility that would come with some of the restructuring would be useful when we start looking at 3d, etc.

[ansis]

duplicate of #146