This separates the action of requesting an extent to be loaded from the action of requesting cached features. The renderer (or any other consumer of a vector source) calls load to request a data extent. A `featureload` event fires when new features are loaded. The renderer (or any other consumer) separately asks for cached features given an extent. This vector source only loads features once, but this separation will also work with sources that make multiple requests for data in different extents.
This also removes the `data` option from the vector source in favor of a `features` option. Since we no longer have shared data structures for geometries, people can manually create features and pass them to a vector source. The `addFeatures` method is exported as well. This is used to add features to a source that don't have a representation on the "remote" (or server).
This moves the feature cache from ol.layer.Vector to ol.source.Vector. These are the minimum changes required to maintain the existing functionality and make tests pass. More refactoring to come.
In b8a9aeb14e a return was added to `ol.renderer.Layer.prototype.getFeatureInfoForPixel`. The `ol.renderer.canvas.VectorLayer.prototype.getFeatureInfoForPixel` method needs the same.
Now that the ol.renderer.Layer base class has a
getFeatureInfoForPixel method, we have to check whether the
source supports GetFeatureInfo, not the layer renderer.
Previously, if a layer didn't have a `getFeature*ForPixel` type method, the loop count would be decremented. This means that all layers would not be considered. Instead, the callback count should be incremented before requests are made and decremented in the callback.
The floor before setting the transform ensures that we translate by integer pixels for odd and even dimenstions. The round before drawing the image ensures pixels are aligned.
Rendering performance degrades as the context lineWidth increases when calling lineTo many times for the same pixel coordinate. This appears to be a widespread performance issue and is very clearly improved by only calling lineTo when in a new pixel coordinate.
