The operation is sketchy for a number of reasons:

1) Mathematically, it doesn't make sense. The code interprets the
QVector3D as a point, extending it with w=1, and uses it as a
row vector. But similarly, the vector could be intepreted as a
directional vector, with w=0. No semantic is "better" than the
other.

2) QMatrix4x4 is not meant to be post-multiplied. Granted,
one could use a QMatrix4x4 as arbitrary storage for 16 floats,
but QMatrix4x4 builds itself to be always *pre* multiplied
(e.g. translate changes the 4th column, not the 4th row). We
can keep post multiplication for the general case if we do it
against a QVector4D, but I don't feel that we should support it
also for QVector3D.

[ChangeLog][QtGui][QMatrix4x4] The multiplication operator
(operator*) between a QVector3D and a QMatrix4x4 has been deprecated.
User code needs to extend the QVector3D to a QVector4D first
(by specifying the intended w coordinate), and then multiply
the QVector4D by the matrix.

Change-Id: I41b64d8ab7eb6126dc4c49fe29cf3f1b7afc7987
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 654a2164

)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>