- Metal's built-in blit based mipmap generator doesn't use box
  filtering. Hence manual generation using compute is needed.

- Compute based mipmap gen can generate up to 4 mips per pass if the
  base level is power of 2.

- This approach can be extended to 2D/cube texture's mipmap generation
  in future.

Bug: angleproject:4921
Bug: angleproject:2634
Change-Id: I7f997669fe39afef075b2bca2406e9424cbb3016
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2336120Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>