The Buffers are exclusively hold by ShmPool. The user of a Buffer is not
supposed to delete it as a no longer used Buffer should be reused by the
ShmPool.
To make it obvious that the ownership of the pointer is not passed to the
user the return type is changed to QWeakPointer. This also allows the
ShmPool to destroy Buffers as needed.
The unit test found a few problems which are now addressed
* getBuffer did not check the format when reusing a buffer
* creatBuffer used the wrong method on QSize to check whether it is empty
* destroy didn't call destroy on the Buffer. This is now added by moving
the Buffer::Private in a dedicated header which can also be included
from the ShmPool
