Change meaning of offset in QByteArrayData

Change meaning of offset in QByteArrayData

It used to be an index into the first element in 'd' that came after
'offset'. It is now the byte offset from the beginning of the
QByteArrayData structure.

By no longer using an actual array to access characters, we also steer
clear of GCC bug #43247:

    http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

This aligns this data structure with QArrayData. The intention is to
have QVector, QString and QByteArray share the same memory layout and
possibly code.

Change-Id: I8546e5f51cd2161ba09bd4ada174b7f5e6f09db7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>