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/Image/Image_Diff.cxx

@@ -288,6 +288,12 @@ Standard_Integer Image_Diff::Compare()
     return -1;
   }
 
+  // first check if images are exactly teh same
+  if (! memcmp (myImageNew->Data(), myImageRef->Data(), myImageRef->SizeBytes()))
+  {
+    return 0;
+  }
+
   // Tolerance of comparison operation for color
   // Maximum difference between colors (white - black) = 100%
   Image_ColorXXX24 aDiff = {{255, 255, 255}};