Qt Tablet Support doesn't work in multimonitor setup on Windows

Setup: 2 monitors in "extend" mode, 1 Wacom tablet mapped to one of the monitor in the driver.

Expected: the events of the tablet are mapped to one of the monitors

Actual: mouse events are coming in correct positions, tablet events are coming in wrong positions, as if they are mapped to two monitors at the same time.

The problem is that Qt uses default wintab context instead of the current one (that is it uses ptrWTInfo(WTI_DEFCONTEXT, 0, &lcMine), instead of correct ptrWTGet(hTab, &lc)). The latter must be used.

Here is the diff we used to fix the problem locally:
http://paste.kde.org/pysrdjznw

I didn't check whether the code is correct in Qt5 or not.