mesa: don't expose GL_ARB_fragment_coord_conventions until the GLSL part is done

Exposing it was incorrect, as the GLSL part of the extension is
missing.

We still keep the ARB_fragment_coord_conventions field, so that the
ARBfp parser can know whether to accept or reject the keywords.

src/mesa/main/extensions.c

@@ -50,7 +50,8 @@ static const struct {
    { OFF, "GL_ARB_depth_clamp",                F(ARB_depth_clamp) },
    { ON,  "GL_ARB_draw_buffers",               F(ARB_draw_buffers) },
    { OFF, "GL_ARB_draw_elements_base_vertex",  F(ARB_draw_elements_base_vertex) },
-   { OFF, "GL_ARB_fragment_coord_conventions", F(ARB_fragment_coord_conventions) },
+   /* TODO: uncomment the following line once GLSL layout(...) support is implemented */
+   /* { OFF, "GL_ARB_fragment_coord_conventions", F(ARB_fragment_coord_conventions) }, */
    { OFF, "GL_ARB_fragment_program",           F(ARB_fragment_program) },
    { OFF, "GL_ARB_fragment_program_shadow",    F(ARB_fragment_program_shadow) },
    { OFF, "GL_ARB_fragment_shader",            F(ARB_fragment_shader) },