Use QLastCursorPosition to check for missing cursor position

After c5792dcfd631abb4f9e2b92cd6e88d7e5c373406 the last cursor position
is reflected as a QPoint(std::numeric_limits<int>::max(), ...), and
after 209a2145f94e99f99832c3a08cdf579d8f42ca55 as 1 << 23. Instead of
hard coding the sentinel value, let's use QLastCursorPosition directly.

Pick-to: 6.9 6.8 6.5
Change-Id: I24031f3cc800d2f0028eb95f4f8597019cc945c0
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qsimpledrag.cpp

@@ -195,7 +195,8 @@ void QBasicDrag::startDrag()
     QPoint pos;
 #ifndef QT_NO_CURSOR
     pos = QCursor::pos();
-    if (pos.x() == int(qInf())) {
+    static constexpr QGuiApplicationPrivate::QLastCursorPosition uninitializedCursorPosition;
+    if (pos == uninitializedCursorPosition) {
         // ### fixme: no mouse pos registered. Get pos from touch...
         pos = QPoint();
     }