Don't position cursor inside complex graphemes when clicking outside line

If you clicked to the right of a text line ended with a grapheme consisting of
several characters, either because it was a unicode surrogate pair or a ligature,
we would always to the previous character in logical order, without checking
if this was a valid cursor position.

One result of this would be that hitting backspace when the cursor was positioned
in an invalid position, would cause the entire contents of the document to
become invalid.

Instead we should search backwards for the previous grapheme boundary using
QTextEngine::previousLogicalPosition().

[ChangeLog][QtGui][Text] Fixed position of text cursor set by clicking outside
the bounds of a text line that ends with a surrogate pair or ligature.

Task-number: QTBUG-69085
Change-Id: I7224c065f332f398bdfbb3f42b2b3ec8280c76a4
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/gui/text/qtextlayout.cpp

@@ -3002,9 +3002,8 @@ int QTextLine::xToCursor(qreal _x, CursorPosition cpos) const
     // character between lines is a space and we want
     // to position the cursor to the left of that
     // character.
-    // ###### breaks with japanese for example
     if (this->index < eng->lines.count() - 1)
-        --maxPos;
+        maxPos = eng->previousLogicalPosition(maxPos);
 
     pos = qMin(pos, maxPos);
     return pos;