The ES3 spec and this extension lead to some awkward interactions. The
extension doesn't mention allowed filtering modes, so be default it
allows both linear and nearest. ES3 however, very clearly restricts
any depth texture (any texture with effective internal format which
has depth or stencil bits) to only use nearest filtering. This then
breaks compatibility with the legazy unsized formats.

Choose to be slightly non-conformant here, and allow linear filtering
for the older unsized formats. Although this could lead to problems
down the line, it is consistent with existing ES drivers in practice.

Other future options might be to override the behaviour of the filter
to use nearest when using WebGL validation compatibility.

BUG=chromium:649200

Change-Id: I4ee7608dd04d1fd238385aefee32c8c9c1e80ca0
Reviewed-on: https://chromium-review.googlesource.com/388759Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>