Vulkan: __samplerExternal2DY2YEXT-aware TextureVk This CL adds the ability for TextureVk to return an ImageView that is created with a VkSamplerYcbcrConversion object that uses an identity conversion model. This allows direct sampling of YUV values without RGB conversion, which is needed for __samplerExternal2DY2YEXT. Bug: b/223456677 Change-Id: Ie1d4e12375b7808a1f060747bc2d74baeda3fdea Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3751889 Commit-Queue: Lingfeng Yang <lfy@google.com> Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>