- Support default integer attributes.
- When converting index buffer, use offset modulo instead of offset to
  reduce number of conversions if application uses many different
  offsets to the index buffer.

Bug: angleproject:2634
Change-Id: I97aa9ea1826ffc9dbe5784fe5b5af2f99df63e2c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2494524
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>