Prior to this change, the transform feedback buffer offset was not
stored in the descriptor set key, so if the offset changed, stale
descriptor sets could be used.

Bug: angleproject:5963
Change-Id: I3dec4ab9fa82092a65e9a75bdd19c5f2cf49521c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2894513Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>