texture-rectangle: Initialize is_foreign

So we don't read an initializes value later on. Caught by valgrind:

Conditional jump or move depends on uninitialised value(s)
  _cogl_object_texture_rectangle_indirect_free (cogl-texture-rectangle.c:105)
  _cogl_object_context_indirect_free (cogl-context.c:453)
  ...
  main (text.c:149)

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Neil Roberts <neil@linux.intel.com>