IECoreArnoldPreview : fix ambiguous comparison for MSVC
IECoreArnoldPreview : fix ambiguous comparison for MSVC

- Fixes MSVC error :
    gaffer\src\iecorearnold\renderer.cpp(956): error C2679: binary '==': no operator found which takes a right-hand operand of type 'IECore::InternedString' (or there is no acceptable conversion)
    c:\program files (x86)\windows kits\10\include\10.0.17763.0\shared\guiddef.h(192): note: could be 'bool operator ==(const GUID &,const GUID &)'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\thread(208): note: or       'bool std::operator ==(std::thread::id,std::thread::id) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\random(2572): note: or       'bool std::operator ==(const std::bernoulli_distribution &,const std::bernoulli_distribution &)'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\variant(1653): note: or       'bool std::operator ==(std::monostate,std::monostate) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\system_error(284): note: or       'bool std::operator ==(const std::error_condition &,const std::error_condition &) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\system_error(278): note: or       'bool std::operator ==(const std::error_condition &,const std::error_code &) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\system_error(272): note: or       'bool std::operator ==(const std::error_code &,const std::error_condition &) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\system_error(266): note: or       'bool std::operator ==(const std::error_code &,const std::error_code &) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\exception(331): note: or       'bool std::operator ==(const std::exception_ptr &,std::nullptr_t) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\exception(326): note: or       'bool std::operator ==(std::nullptr_t,const std::exception_ptr &) noexcept'
    c:\program files (x86)\microsoft visual studio\2017\community\vc\tools\msvc\14.16.27023\include\exception(321): note: or       'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &) noexcept'
    gaffer\include\boost\filesystem\path.hpp(763): note: or       'bool boost::filesystem::operator ==(const boost::filesystem::path::value_type *,const boost::filesystem::path &)'
    gaffer\include\boost\filesystem\path.hpp(762): note: or       'bool boost::filesystem::operator ==(const boost::filesystem::path &,const boost::filesystem::path::value_type *)'
    gaffer\include\boost\filesystem\path.hpp(761): note: or       'bool boost::filesystem::operator ==(const boost::filesystem::path::string_type &,const boost::filesystem::path &)'
    gaffer\include\boost\filesystem\path.hpp(760): note: or       'bool boost::filesystem::operator ==(const boost::filesystem::path &,const boost::filesystem::path::string_type &)'
    gaffer\include\boost\filesystem\path.hpp(759): note: or       'bool boost::filesystem::operator ==(const boost::filesystem::path &,const boost::filesystem::path &)'
    gaffer\src\iecorearnold\renderer.cpp(956): note: while trying to match the argument list '(const std::string, IECore::InternedString)'

hradec/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,hradec/gaffer,hradec/gaffer