Section 4.3.7 (Interpolation) of the GLSL 1.30 spec says:

    "If gl_Color is redeclared with an interpolation qualifier, then
    gl_FrontColor and gl_BackColor (if they are written to) must also be
    redeclared with the same interpolation qualifier, and vice versa. If
    gl_SecondaryColor is redeclared with an interpolation qualifier,
    then gl_FrontSecondaryColor and gl_BackSecondaryColor (if they are
    written to) must also be redeclared with the same interpolation
    qualifier, and vice versa. This qualifier matching on predeclared
    variables is only required for variables that are statically used
    within the shaders in a program."

This adds five sets of tests from a single generator script:

    - Set an interpolation qualifier, possibly the default, for one of
      gl_FrontColor, gl_BackColor, gl_FrontSecondaryColor, or
      gl_BackSecondaryColor in the vertex shader, and set a different
      interpolation qualifier, possibly the default, for the matching
      gl_Color or gl_SecondaryColor in the fragment shader.  This should
      fail to link.

    - Set a non-default interpolation qualifier for one of
      gl_FrontColor, gl_BackColor, gl_FrontSecondaryColor, or
      gl_BackSecondaryColor in the vertex shader, and have no
      redeclaration in the fragment shader.  In the fragment shader,
      neither gl_Color nor gl_SecondaryColor is used.  This should
      successfully link.

    - Set a non-default interpolation qualifier for one of gl_Color or
      gl_SecondaryColor in the fragment shader, and have no
      redeclaration in the vertex shader.  In the vertex shader, none of
      the built-in color variables are used.  This should successfully
      link.

    - Set an interpolation qualifier, possibly the default, for one of
      gl_FrontColor, gl_BackColor, gl_FrontSecondaryColor, or
      gl_BackSecondaryColor in the vertex shader, and set a different
      interpolation qualifier, possibly the default, for the matching
      gl_Color or gl_SecondaryColor in the fragment shader.  Neither
      stage uses any of the color variables.  This should successfully
      to link.

    - Set conflicting interpolation qualifiers for gl_FrontColor /
      gl_BackColor or gl_FrontSecondaryColor / gl_BackSecondaryColor in
      the vertex shader, and set a matching interpolation qualifier on
      gl_Color or gl_SecondaryColor in the fragment shader for one of
      the vertex shader built-in varaibles.  This should fail to link.

On Mesa, all of the tests that expect linking failures fail.  Basically,
Mesa's linker accepts everything.

NVIDIA (304.64 on GTX 260) fails the same set of tests.

v2: Use Python lists instead of "set literal" syntax (Python 2.7+ only).
Change the names of several maps to include the suffix _map.  Rename
some additional varibles that had a misleading "front" or "back" in the
name.  All changes suggested by Paul.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47755


Reviewed-by: Paul Berry <stereotype441@gmail.com>