Don't clear line rendition on EL and partial ED

EL (Erase Line) should not reset the line rendition attribute to
single-width. ED (Erase Display) should only reset it for completely
cleared lines. ECH (Erase CHaracters) should obviously not reset it.
DECSED and DECSEL (Selective Erase, not supported by Konsole) should not
reset it.

This fixes a vttest test where a line is set to double-height-top and
then EL before writing its text and the double-height-bottom line below.

Can also be tested with:

echo -e "\e[2J\e[4;1HNormal\n\e#6DOUBLE\n\e#6DOUBLE\nNormal" &&
sleep 2; echo -e "\e[5;3H\e[1J\e[8;1H"