GUAC-717: Add GUAC_CHAR_CONTINUATION sentinel. Add width property to characters.
GUAC-717: Set continuation character for wide chars as necessary.
GUAC-717: Clear up to overwritten character start when setting characters.
GUAC-717: Clear up to character end when setting characters. Handle character overwrite as part of copy.
GUAC-717: Handle each logical subregion of a copy as a separate overwrite.