Don't select half a CJK character
This was a visual regression. Code using the selection already takes care of dealing with CJK wide characters, and the old rendering in TerminalPainter also did. The new rendering is split in several layers, which need to know about the selection to use the proper background and foreground colors, and this caused the regression.
To prevent trouble, we now take care of CJK wide characters before that, when obtaining the characters from the model (Screen), were characters are marked as selected if they fall into the selection coordinates.