fix wrapping at bottom of pane

[aaronjanse]

By definition, we're at the bottom of the scrolling region when v.Cursor.Y == v.scrollingRegion.bottom - 1:

3mux/vterm/vterm.go

Lines 16 to 17 in e2945b0

	// cursor within when cursor.Y < bottom
	bottom int

This fixes related off-by-one error. It also tells 3mux to scroll when wrapping at the bottom right corner of a scrolling region.

Test case, when run in a shell with a prompt at the bottom of a pane:

seq 1 1 100 | paste -s -d+

[PotatoParser]

Looks great! Working as intended!