Parallelize CPU intensive loops

[atomashpolskiy]

The original version uses OpenMP for parallelization, in 6 places in total. So far I've been able to use Rayon to modify the corresponding code in the Rust version, in 2 places:

(1)
MLPLayer::Compute (src/classifier/mlp.cpp:39)
classifier::surf_mlp_classifier::Layer::compute (src/classifier/surf_mlp_classifier.rs:153)

(2)
SURFFeatureMap::ComputeGradY (src/feat/surf_feature_map.cpp:197)
feat::surf_mlp_featmap::SurfMlpFeatureMap::compute_grad_y (src/feat/surf_mlp_featmap.rs:137)

Here are the remaining places:

(3)
SURFFeatureMap::ComputeGradX (src/feat/surf_feature_map.cpp:178)
feat::surf_mlp_featmap::SurfMlpFeatureMap::compute_grad_x (src/feat/surf_mlp_featmap.rs:116)

(4)
LABFeatureMap::ComputeRectSum (src/feat/lab_feature_map.cpp:145)
feat::lab_boosted_featmap::LabBoostedFeatureMap::compute_rect_sum (feat/lab_boosted_featmap.rs:183)

(5)
LABFeatureMap::ComputeFeatureMap (src/feat/lab_feature_map.cpp:173)
feat::lab_boosted_featmap::LabBoostedFeatureMap::compute_feature_map (feat/lab_boosted_featmap.rs:215)

(6)
fd::ResizeImage (include/util/image_pyramid.h:44)
common::resize_image (common/image_pyramid.rs:175)