- If BufferMtl is static or large size, don't use shadow copy. Use one
  MTLBuffer and map directly on it.
- If BufferMtl is dynamic and small size, use shadow copy and buffer
  pool of 10 MTLBuffer (s). The MTLBuffer is allocated in shared memory
  in this case (PCI-E memory for example). MTLBuffer in shared memory
  region doesn't need to sync content between CPU and GPU.
- When copyBuffer, if BufferMtl is being used by GPU use blit command to
  do the copy on GPU side.
- Also implemented GL_MAP_UNSYNCHRONIZED_BIT.

Bug: angleproject:2634
Change-Id: I7a5aab309d24c76106a7087358ee5883ee05d250
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2408592
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>