Properly reset the dirty flag

Currently, the dirty flag is never reset (to false).  This is a bug.  Because renderFrame is called very often (every layer render gets called when every other layer needs to re-render), it is criticial to know when we can bail out early.  The dirty flag is currently the way that the vector layer renderer knows that it needs to do more work.  On an empty cache, the renderFrame method of the vector layer renderer is called ~30 times for a single zoom in the vector layer example (due to tiles loading on other layers).  Without this change, we miss the fast path out and clear/re-render the canvas all 30 times.  With this change, we are only clear the canvas and redraw 6 times in a typical zoom animation.
Merge pull request #718 from tschaub/dirty-frame

Properly reset the dirty flag
More sophisticated RTree implementation

This new implementation is based on
http://github.com/imbcmdth/RTree/, with only a few modifications
to add the optional type and provide the API of the previous
implementation.

There is still room for optimization, but this is such an
improvement over the previous RTree already that it's worth
bringing it in now.
Only query RTree if requested geometryType is used
Use strings, like with other indices
Making RTree work in advanced compilation mode
Better optimization for advanced compilation mode
More performance and size optimizations

Not using Math.max and Math.min increases performance, and by
using ol.extent functions instead of RTree's rectangle
structures and calculations we can get rid of several functions.
Making docs more clear
Porting tests from http://github.com/imbcmdth/RTree/

Also fixes some issues that were revealed by the new tests.
Pass layers as array instead of ol.Collection
Merge pull request #721 from fredj/cleanup

Pass layers as array instead of ol.Collection
Addressing @tschaub's review comments
Merge pull request #719 from ahocevar/fast-rtree

More sophisticated RTree implementation. r=@tschaub
Export addFeatures

This needs discussion, but it's useful for now to be able to
have applications with programmatically created features.
Merge pull request #723 from ahocevar/add-features

Export addFeatures. r=@tschaub