Support stencil texturing in WebGL Updated WebGL sampler format validation to support sampling from stencil textures and combined depth/stencil textures in stencil mode. Bug: angleproject:3231 Bug: angleproject:8051 Change-Id: Ib35dfd91f7ce77fc50bd749f49602ffe86837c67 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4315864 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>