glsl: subroutine types cannot be compared

glsl: subroutine types cannot be compared

subroutine variables are to be used just in the way functions are
called. Although the spec doesn't say it explicitely, this means that
these variables are not to be used in any other way than those left
for function calls. Therefore, a comparison between 2 subroutine
variables should also cause a compilation error.

From The OpenGL® Shading Language 4.40, page 117:

  "  To use subroutines, a subroutine type is declared, one or more
     functions are associated with that subroutine type, and a
     subroutine variable of that type is declared. The function
     currently assigned to the variable function is then called by
     using function calling syntax replacing a function name with the
     name of the subroutine variable. Subroutine variables are
     uniforms, and are assigned to specific functions only through
     commands (UniformSubroutinesuiv) in the OpenGL API."

From The OpenGL® Shading Language 4.40, page 118:

  "  Subroutine uniform variables are called the same way functions
     are called. When a subroutine variable (or an element of a
     subroutine variable array) is associated with a particular
     function, all function calls through that variable will call that
     particular function."

Fixes GL44-CTS.shader_subroutine.subroutines_cannot_be_assigned_float_int_values_or_be_compared

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>