fix cursor visible issue

fix cursor visible issue

If the height of the cursor(caret) is bigger than the text control, the cursor is not visible.
This seems to be the intended behaviour.
But the user expects the cursor to be clipped even if the text control is smaller than the cursor.

So I added mCursorLayer for cursor clipping.
Because the mActiveLayer has to draw grab and selection handles outside of the text control,
so it is inappropriate to clip.

Change-Id: Id1510bca2e45e5efa091295ea798cce482e61012
Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>