- When converting vertex buffer:
  - if there is no render pass active, use compute shader to convert.
  - if there is a render pass active and device supports explicit memory
    barrier then convert the buffer in vertex shader with direct buffer
    write and insert a memory barrier.
  - if there is a render pass active and device doesn't support explicit
    memory barrier then convert the buffer on CPU.

Bug: angleproject:2634
Change-Id: I5346e3a2adb855f40e46a3912d9db404a4482e0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2434025
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>