Initialize QGuiApplicationPrivate::lastCursorPosition to inf

This makes sure that we don't think a mouse area at position 0, 0
contains the mouse on startup.

Task-number: QTBUG-36191

Change-Id: I68a8aed195531f506372ef465734661069b60f57
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>