[segmentation] Make RegionGrowingRGB faster

[MaximSmolskiy]

Resolve #6356

For deterministic comparison of implementations I made the following changes:

--- a/segmentation/include/pcl/segmentation/impl/region_growing.hpp
+++ b/segmentation/include/pcl/segmentation/impl/region_growing.hpp
@@ -614,7 +614,7 @@ pcl::RegionGrowing<PointT, NormalT>::getColoredCloud ()
   {
     colored_cloud.reset(new pcl::PointCloud<pcl::PointXYZRGB>);
 
-    srand (static_cast<unsigned int> (time (nullptr)));
+    srand (static_cast<unsigned int> (0));
     std::vector<unsigned char> colors;
     for (std::size_t i_segment = 0; i_segment < clusters_.size (); i_segment++)
     {
@@ -664,7 +664,7 @@ pcl::RegionGrowing<PointT, NormalT>::getColoredCloudRGBA ()
   {
     colored_cloud.reset(new pcl::PointCloud<pcl::PointXYZRGBA>);
 
-    srand (static_cast<unsigned int> (time (nullptr)));
+    srand (static_cast<unsigned int> (0));
     std::vector<unsigned char> colors;
     for (std::size_t i_segment = 0; i_segment < clusters_.size (); i_segment++)
     {

and replaced std::sort with std::stable_sort for old implementation

Total reg.extract (clusters) time decreased from 16 to 4 seconds and applyRegionMergingAlgorithm time - from 12 to 0.3 seconds