Viz initial backport, compilation fixes, moved viz/viz.hpp header according to 2.4 style

modules/viz/test/test_precomp.cpp

@@ -0,0 +1,24 @@
+#include "test_precomp.hpp"
+
+cv::String cv::Path::combine(const String& item1, const String& item2)
+{
+    if (item1.empty())
+        return item2;
+
+    if (item2.empty())
+        return item1;
+
+    char last = item1[item1.size()-1];
+
+    bool need_append = last != '/' && last != '\\';
+    return item1 + (need_append ? "/" : "") + item2;
+}
+
+cv::String cv::Path::combine(const String& item1, const String& item2, const String& item3)
+{ return combine(combine(item1, item2), item3); }
+
+cv::String cv::Path::change_extension(const String& file, const String& ext)
+{
+    String::size_type pos = file.find_last_of('.');
+    return pos == String::npos ? file : file.substr(0, pos+1) + ext;
+}