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.
To avoid surprises for application developers, this change
creates a new getFeatures method. So it is clear now beforehand
whether features or feature info markup is returned. The result
is now also grouped by layer, so application developers always
have a link between a layer and the feature info it returns.
To make getFeatureInfo return markup for vector layers, this
change also adds a featureInfoFunction property to the vector
layer, which gives developers full control over how features are
rendered to feature info markup.
Previously, the map renderer would listen for layers being added and
removed from the layers collection, and would create and remove layer
renderers in response to these events.
With this change, layer renderers are only created or removed when
renderFrame is called, which leads to somewhat simpler code.
We still need to listen to changes to the layers collection, but now
these only trigger a new render.
This new approach also has an advantage when layers change order.
Swapping the order of two layers involves removing one and re-inserting
it elsewhere. With the old approach, this would cause the deletion and
re-creation of the layer renderer. With this new approach, the layer
renderer is preserved.
With the frameState stuff the map renderer need not listen to view changes. The map renderer receives orders from the map. These orders are renderFrame_ calls with frameState objects representing the current state.
