In combination with a (not yet implemented) `Symbolizer#isLiteral` method, calling `Symbolizer#evaluate` without a feature is the fast track for rendering batches of like-styled features. This change also simplifies the handling of undefined symbolizer literal properties (where stroke or fill properties are optional).
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.
Now vector layers can have a style. ol.Style instances have an
apply method to get the symbolizer literals for a feature. If the
layer does not have a style defined, there is also a static
applyDefaultStyle function on ol.Style to get the default
symbolizer literals for a feature. The vector layer also got a
groupFeaturesBySymbolizerLiteral method, which returns an array
with features grouped by symbolizer, as needed by the canvas
renderer.
