minor stitching optimization (use cpu matchers instead of gpu matchers on multicore processors)

modules/stitching/matchers.cpp

@@ -411,7 +411,15 @@ namespace
 
 BestOf2NearestMatcher::BestOf2NearestMatcher(bool try_use_gpu, float match_conf, int num_matches_thresh1, int num_matches_thresh2)
 {
+    bool use_gpu = false;
     if (try_use_gpu && getCudaEnabledDeviceCount() > 0)
+    {
+        DeviceInfo info;
+        if (info.majorVersion() >= 2 && cv::getNumberOfCPUs() < 4)
+            use_gpu = true;
+    }
+
+    if (use_gpu)
         impl_ = new GpuMatcher(match_conf);
     else
         impl_ = new CpuMatcher(match_conf);