Added buffer copy command and support for VkBufferCopy regions.

Passes all tests in:
dEQP-VK.api.copy_and_blit.core.buffer_to_buffer

Bug b/118619338

Change-Id: Id69cd3f089b07343a7209290808c11d4c17f1a49
Reviewed-on: https://swiftshader-review.googlesource.com/c/23229Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>