EGL1.5 spec says depth/stencil data are undefined after swap. This CL
will track the depth/stencil data and mark it as undefined and skip the
load if it is undefined.

Bug: b/153885625
Change-Id: Ifb3d88d442da547fa78c1eae091cbae08a20d0ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2148179Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>