If a PBO is bound when uploading the initialization data, GL erorrs or crashes
can happen.

Covered by tests added in
https://chromium-review.googlesource.com/c/angle/angle/+/1764476

BUG=angleproject:3858

Change-Id: I16963b1c09b5b173665a87f77296607f8639c975
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1769058Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>