Add some missing 'const' and texture-op guards, for future change to enum-based textu...

Add some missing 'const' and texture-op guards, for future change to enum-based texture calls.

The infrastructure is in place to not do text comparisons for "texture" ... for deducing type of
texture call.  But, it is not yet turned on, as it could break some consumers.  Am soliciting
any feedback on that.

See in Initialize.cpp: const bool PureOperatorBuiltins = false;  // could break backward compatibility; pending feedback