Buffer now has the ability to copy its content to or from memory.

Bug b/118383648

Change-Id: Ic01bf049fe054559299830ca750263c2277e4d86
Reviewed-on: https://swiftshader-review.googlesource.com/c/23069Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>