VS: Use case-insensitive check for hlsl,jpg,png,xml file extensions

These "extra" sources should map to the proper tool even when they
are not in lower case.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1130,7 +1130,7 @@ void cmVisualStudio10TargetGenerator::WriteExtraSource(cmSourceFile const* sf)
   bool toolHasSettings = false;
   std::string tool = "None";
   std::string shaderType;
-  std::string const& ext = sf->GetExtension();
+  std::string ext = cmSystemTools::LowerCase(sf->GetExtension());
   if(ext == "hlsl")
     {
     tool = "FXCompile";