Use special value SMOOTH_SCALABLE where appropriate

It is the same as USHRT_MAX but mentioning a special value makes
the code more readable.

Change-Id: I91063e472a6130ceb47f866344709786e4b05f20
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Fix assertion introduced in 8e3fdf13543f43d7a4cb4fa41f7279f9a5d274d3

It is clear from the commit itself that `m_engines[1]` isn't accessible
after `m_engines.resize(1)`.

Change-Id: I7b3977cca3f3aeaabadb5ff4f3e52a418022123c
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>
Auto-scroll while selecting entire rows/columns did not work

If you press and hold a section in a header view you can extend the
selection to more rows by moving the mouse. This worked fine until you
moved the mouse outside the geometry of the header view. The expected
behavior was then to scroll the view (this is what happens with extended
selections on regular table cells).

[ChangeLog][QtWidgets][QHeaderView] Auto-scroll the view when making
extended row/column selections.

Change-Id: Ic65aa34d370e74054b2123ab57edb1add0e8adb9
Task-number: QTBUG-21201
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>