mesa: add gl_texture_image::Face, Level fields

Several drivers have these fields in their subclasses of gl_texture_image.
They'll be useful for core Mesa too...

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/teximage.c

@@ -571,6 +571,8 @@ _mesa_set_tex_image(struct gl_texture_object *tObj,
 
    /* Set the 'back' pointer */
    texImage->TexObject = tObj;
+   texImage->Level = level;
+   texImage->Face = face;
 }