When the packed stride and source stride were different, the streaming
code path used a loop that added two times the offset for the start of
the index range.

BUG=angleproject:1135

Change-Id: I6d314a1a28abec4df22f7c798c1af87a18d8b7ec
Reviewed-on: https://chromium-review.googlesource.com/295225Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>