VLFeat compatibility

[griwodz]

Description

This PR delivers code that is identical to PR #112, but the changes have been rearranged into different commits to make examining the changes easier.

The proposed change adds a descriptor computation that is compliant with VLFeat. The original PopSift descriptor was not because it used the angle differently: while VLFeat spreads the angular part of the descriptor over up to 8 bins of the descriptor's histogram, PopSift would only spread it over two. Furthermore, descriptors are arranged differently into the histograms (according to VLFeat documentation, VLFeat does that also differently from Lowe's original code).

Features list

For backward compatibility, the VLFeat descriptor is not the default descriptor. The following settings lead to results that seem to be identical to the VLFeat 0.9.20 C code and command line (the 0.9.21 command line does not work):

--desc-mode=vlfeat              : use the VLFeat descriptor extraction
--norm-mode=classic             : use L2 normalization, not RootSift
--norm-multi 9 --write-as-uchar : multiply descriptor values with 2^9 and cast to uchar
--write-with-ori                : output of descriptors in the same format as VLFeat command line (instead of OpenCV-compliant)
--initial-blur 0.5              : PopSift default
--sigma 1.6                     : PopSift default
--threshold 0                   : set the peak threshold to 0
--edge-threshold 10.0           : PopSift default

Implementation remarks

The L2 normalization had a bug that was removed.

The threshold for accepting an orientation in VLFeat is computed before smoothing the histogram, PopSift did it after computing the histogram (consequently accepting fewer orientations). This was change to work like VLFeat.