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.
This is a first draft. The way geometry changes are handled for
now is a bit clumsy. Both updating the feature cache RTree and
making the layer aware of feature and geometry changes could be
handled in a smarter way if these changes would be made through
the layer instead of directly on the geometry or feature.
