Basic infrastructure for redeclaring built-in variables (like gl_FragCoord) so shaders can change their type/layout.

A modifiable copy is made at the shader's global scope.


git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@23103 e7fa87d3-cd2b-0410-9028-fcbf551c1848