Added an implementation of the Fast Guided Filter

derived directly from the pseudo-code of the paper:

 Fast Guided Filter
 by Kaiming He, Jian Sun

available at https://arxiv.org/abs/1505.00996

rtengine/boxblur.h

@@ -35,6 +35,8 @@ namespace rtengine
 template<class T, class A> void boxblur (T** src, A** dst, int radx, int rady, int W, int H)
 {
     //box blur image; box range = (radx,rady)
+    radx = min(radx, W-1);
+    rady = min(rady, H-1);
 
     AlignedBuffer<float>* buffer = new AlignedBuffer<float> (W * H);
     float* temp = buffer->data;