0025907: Optimization of testdiff command

- Work with strings optimized in Tcl procedures used in testdiff command
- CPU and memory differences output of testdiff command improved to give relative change in percents
- Cumulative CPU and memory differences are output for test grids
- In HTML output of testdiff command, deviations of memory and CPU measurements greater than 5% are colored (red or green)
- Search of image files in testdiff command corrected to avoid wrong attribution of image file to issues starting with the same first letters; images must start with the test case name, optionally followed by underscore or dash and arbitrary text
- Image_Diff tool optimized for the case if images are exactly the same
- Perf_Meter class output corrected, destructor made non-virtual
- DRAW command diffimage optimized to not save diff files if there is no difference
- Tests User Guide updated according to these changes and actual state

src/OSD/OSD_PerfMeter.cxx

@@ -241,7 +241,7 @@ void perf_sprint_all_meters (char *buffer, int length, int reset)
   for (i=0; i<nb_meters; i++) {
     const t_TimeCounter * const ptc = &MeterTable[i];
     if (ptc && ptc->nb_enter) {
-      int n = sprintf (string, "          Perf meter results               :   enters  seconds  sec/enter\n");
+      int n = sprintf (string, "          Perf meter results               :   enters  seconds  microsec/enter\n");
       if (n < length)
       {
         memcpy (buffer, string, n);

src/OSD/OSD_PerfMeter.hxx

@@ -59,7 +59,7 @@ public:
   void Flush() const { perf_close_imeter(myIMeter); }
 
   //! Assures stopping upon destruction
-  virtual ~OSD_PerfMeter() { if (myIMeter >= 0) Stop(); }
+  ~OSD_PerfMeter() { if (myIMeter >= 0) Stop(); }
 
 protected: