Vulkan: add support for shadow samplers. Adds support for ES 3.0 shadow samplers. Test: ./angle_deqp_gles3_no_gtest --deqp-egl-display-type=angle-vulkan -n 'dEQP-GLES3.functional.texture.shadow.2d.*' Bug: angleproject:3211 Change-Id: Ic82ecfe19290fb952fcb0ba423691b5bac87c4a3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1652021 Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>