[Reg: 5.6 -> 5.7] Wrong position in mousePressEvent and mouseMoveEvent for Wacom pen tablets

Mouse press events carry the position of the mouse pointer at the time when the event is delivered and not where it originally occurred. If the application is busy when the user presses a mouse button and the mouse is moved after pressing the mouse button, this leads to mousePressEvents at locations where the user never pressed a mouse button. This only affects mousePressEvents coming from a Wacom pen tablet. Events from regular mice seem to behave as expected.

In the attached simple example:

• Click quickly about 10 different positions in the Window using a Wacom tablet with pen, then rest the mouse pointer at the last position.
• With Qt 4.x, 5.4, 5.5, 5.6, the 10 clicked positions are correctly marked with a circle (the positions are also printed to stdout).
• With Qt 5.7, the first one or two points appear about at the correct position, all other clicks are registered on top of each other at the position where the mouse is currently resting. This creates mousePressEvents at locations where the user never clicked.

This might be a regression of:
https://bugreports.qt.io/browse/QTBUG-40642
In Qt 5.7, only clicks from Wacom pen tablets are affected though.