Wrong length was given for the remainder highlight attribute,
which caused the highlight to overflow.

Steps to reproduce:
1. [English] type abcdefgh and move cursor to middle of text abcd|efgh
2. [Japanese] enter word "hiragana"
3. [Japanese] press left arrow ←

Observed result:
Pre-edit text highlight (light blue) exceeds the pre-edit text.

Expected result:
Pre-edit text highlight should not exceed the pre-edit text.

Change-Id: I66002357e2fc008ccdd6f89db8baaab64f9d5070
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>