Reassign cursor position for certain cases

The current cursor position assignment is not ideal for bi-directional text in the following case (suppose abc is LTR text and ABC is RTL text).

1. Given a string "abcABCabc":

1) If the cursor position is 3, the actual layout will show as "abcCBA|abc", where '|' represents the cursor;
2) If the cursor position is 6, the actual layout will also show as "abcCBA|abc".

There is no way to distinguish these two cases visually, unless we move the cursor again, this visual position has been occupied twice with two different logical positions, which caused ambiguity. Plus, there is no way to move the cursor to make the visual rendering like "abc|CBAabc", that visual position is never occupied.

2. Given a string "ABCabcABC":

1) If the cursor position is 3, the actual layout (right aligned) will show as "CBA|abcCBA";
2) If the cursor position is 6, the actual layout will also show as "CBA|abcCBA".

The same problems as above.

Given the problems above, I propose a reassignment of cursor positions to solve them:

For each QScriptItem with the same direction as the block direction (LTR item in a LTR block, RTL item in a RTL block), if the cursor position equals the starting position of this item, treat it as the end of previous item (QTextEngine::findItem(pos - 1)).

After that, in above case 1., cursor position 6 will appear as "abc|CBAabc", in case 2., cursor position 6 will appear as "CBAabc|CBA", so that cursor position 3 and 6 will not use the same visual position, and the cursor can be placed between the first and second item now.

I think this new assignment makes more sense for bi-directional text and it won't affect any other situations.