GLSL: AttachShader returns INVALID_OPERATION for repeated attach

The GL_ARB_shader_objects spec says that glAttachShaderARB is supposed
to return GL_INVALID_OPERATION if a shader is attached to a program
where it is already attached.  _mesa_attach_shader perviously returned
without error in this case.

src/mesa/shader/shader_api.c

@@ -455,7 +455,13 @@ _mesa_attach_shader(GLcontext *ctx, GLuint program, GLuint shader)
    n = shProg->NumShaders;
    for (i = 0; i < n; i++) {
       if (shProg->Shaders[i] == sh) {
-         /* already attached */
+         /* The shader is already attched to this program.  The
+          * GL_ARB_shader_objects spec says:
+          *
+          *     "The error INVALID_OPERATION is generated by AttachObjectARB
+          *     if <obj> is already attached to <containerObj>."
+          */
+         _mesa_error(ctx, GL_INVALID_OPERATION, "glAttachShader");
          return;
       }
    }