Fix buffer_toscreen() - off-by-one with line wrapping
buffer_linewrap() takes off-by-one line wrapping into account
Only check for line-wrap overflow on non-empty lines

This prevents a negative number in ui_pos.y
Highlight/visual mode works with wrapped lines
region_contains() takes a point_t instead of int x, y
m_dispmove - gj and gk move in display-motions

This is done by iterating over lines, decrementing a count
TODO: g0 and g$