Printing preprocessed shaders (not a supported path): Fix #1490: strings

Put quote marks around strings, due to a change in how tokenization works.

glslang/Public/ShaderLang.h

@@ -532,6 +532,8 @@ public:
         return parse(builtInResources, defaultVersion, ENoProfile, false, forwardCompatible, messages, includer);
     }
 
+    // NOTE: Doing just preprocessing to obtain a correct preprocessed shader string
+    // is not an officially supported or fully working path.
     bool preprocess(const TBuiltInResource* builtInResources,
                     int defaultVersion, EProfile defaultProfile, bool forceDefaultVersionAndProfile,
                     bool forwardCompatible, EShMessages message, std::string* outputString,