With this change, getFeaturesObjectForExtent may return null if
the source does not have data loaded for the provided extent.
A callback can be passed to getFeaturesObjectForExtent to get
notified when the requested data is available.
The canvas vector layer still has the (API candidate) `getFeatures` method that accepts an arbitrary expression (was filter). This, and the `getFeaturesObject` method under it are only used in the tests. The rendering code that was using filters is now calling `layer.getFeaturesObjectForExtent` with an explicit extent and optional geometry type.
The color names are more intuitive. And if we want to support pattern strokes or fills, we'll need additional proerties to represent other pattern properties.
