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>