Remove "init" from VaryingPacking. Instead we can pass the pack mode and size to the collectAndPack method. This cleans up the interface and also allows us to merge two separate code blocks in Program and ProgramPipeline. Bug: angleproject:5496 Change-Id: I390b5d2e8a3b033374ccc5a250597be1f03dec96 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2606531 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>