trace_fixture: Add UpdateClientBufferData2WithOffset. Adds a function to update buffers with an offset. Bug: angleproject:5857 Change-Id: Iddc0dbcdd123866f31c1382e1eb6fc7e7cb922b1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3236997 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>