Vulkan: Let all shader stages share one buffer for default uniform Right now each shader stage has its own vk::DynamicBuffer for default uniform storage. This is less efficient than just share one buffer. This CL moves the storage from per shader stage into its ProgramVk object. Bug: b/159457348 Change-Id: If47248ea23c4e48407d3b211583ae2b048d4d10f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2265281 Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>