vt: implement background color erase (bce) for newline and erase ops
TERM=xterm-256color declares bce in terminfo, so clients like tmux and
htop trust that erasing a cell (EL, ED, ECH, ICH/DCH, IL/DL, and the
blank rows scrolling exposes) fills it with the currently active SGR
background rather than the default — that's why htop's row highlight
stopped short of the right edge instead of reaching it.
Adds a shared blank_cell(g) helper and wires it through clear_row,
erase_line, erase_chars, insert_chars, and delete_chars (which
transitively covers scrolling and insert/delete-line via clear_row).
Also extends newline/NEL/autowrap to pick up an active background for
cells past where the line's writer stopped.
Fixes an ordering bug this surfaced in reset_grid (RIS): it cleared the
screen before resetting cur_bg, so a reset right after setting a
background would wipe to the stale color instead of the true default.