Vulkan: add a feature flag controlling float uniforms 16-bit transform Add a feature flag that will guard whether we want to transform mediump / lowp float uniforms from 32-bit to 16-bit in SpirV. The feature flag is enabled when VK_KHR_16bit_storage extension is supported, and uniformAndStorageBuffer16BitAccess vulkan feature is supported. The feature flag is initialized to false in this change. It will be turned on in a later change when it is being used. Bug: angleproject:405795981 Change-Id: I37c77270fbd391cc97f58890065b66faf77766c1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6852395 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Yuxin Hu <yuxinhu@google.com>