Add #include processing to glslang (though turned off by default).

When an include directive is recognized by the preprocessor, it
executes a callback on the filepath argument to obtain the file
contents.  That way the compilation client can deal with the file
system, include paths, etc.

Currently only accepts quoted filepaths -- no angle brackets yet.

glslang/MachineIndependent/ParseHelper.h

@@ -214,6 +214,7 @@ public:
     int getNumErrors() const { return numErrors; }
     const TSourceLoc& getCurrentLoc() const { return currentScanner->getSourceLoc(); }
     void setCurrentLine(int line) { currentScanner->setLine(line); }
+    void setCurrentColumn(int col) { currentScanner->setColumn(col); }
     void setCurrentSourceName(const char* name) { currentScanner->setFile(name); }
     void setCurrentString(int string) { currentScanner->setString(string); }
     void setScanner(TInputScanner* scanner) { currentScanner  = scanner; }