std::make_unique is C++14, so for strict C++11, we have
to use std::unique_ptr. Added a new utility function,
CommandBuffer::addCommand, to make this change more
readable.

Change-Id: Icc62874a107543fa2c0599a338dbc6676047ad77
Reviewed-on: https://swiftshader-review.googlesource.com/c/23548Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>