Allow writing only half of a double-height line

While DECHDL sequences should be used in pairs on adjacent lines, the
first line drawing the top half, and the second line drawing the bottom
half, apparently real VTxxx (or at least the VT220) terminals did draw
only the top or bottom of characters when confronted with an isolated
DECDHL line (https://retrocomputing.stackexchange.com/questions/18023/).
xterm has that same behavior. So behave likewise.