feat(rasters): Support widget calculations on rasters

[donmccurdy]

In large part, widgets for raster datasources reuse the same code implemented for quadbin tilesets (#50). To accomplish this, we pre-process each raster tile, creating a 'FeatureData' key/value object for each pixel in the raster, if not excluded by the spatial filter. Creating "an object per pixel" is far from ideal, and keeping a binary representation throughout widget calculations would greatly improve performance. However, I think it's better to keep the code aligned with quadbin tilesets right now — ideally any performance optimizations would benefit both. Use of web workers should prevent calculations from dragging down the web application, even if the calculations themselves are inefficient.

Changes:

• Add WidgetRasterSource class
• Add tileFeaturesRaster() function, doing the heavy lifting of extracting feature data for calculations from rasters
• Add raster example
• Minor changes to example widgets, making it easier to set category colors that match pixel colors for debugging

[donmccurdy]

Removing this comment because uniqueIdProperty is inferred automatically in nearly all cases, the user shouldn't need to worry about it. In deck.gl this is a layer property, not a source/data property, and migrating that seems like too much of a breaking change for the benefit.