Enable pbuffers

If a driver does not actually support pbuffer rendering, it can just
not enable any pbuffer fbconfigs.

glx/extension_string.c

@@ -80,7 +80,7 @@ static const struct extension_info known_glx_extensions[] = {
    { GLX(SGI_swap_control),            VER(0,0), N, },
    { GLX(SGIS_multisample),            VER(0,0), Y, },
    { GLX(SGIX_fbconfig),               VER(1,3), Y, },
-   { GLX(SGIX_pbuffer),                VER(1,3), N, },
+   { GLX(SGIX_pbuffer),                VER(1,3), Y, },
    { GLX(SGIX_visual_select_group),    VER(0,0), Y, },
    { NULL }
 };

glx/glxscreens.c

@@ -177,6 +177,7 @@ static char GLXServerExtensions[] =
                         "GLX_SGIX_swap_barrier "
 #endif
 			"GLX_SGIX_fbconfig "
+			"GLX_SGIX_pbuffer "
 			"GLX_MESA_copy_sub_buffer "
 			;