Finish GL_ARB_shading_language_420pack: *ProgramTexelOffset built-in constants.  Also, make comma operation not fold constants.


git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@26602 e7fa87d3-cd2b-0410-9028-fcbf551c1848