vt(4): If the cursor didn't move, don't mark its position as dirty

Currently, this has no effect, because the cursor is always redrawn
anyway. But this will be useful after improvements to the vd_bitbltchr_t
callback API.

The vt_device structure members used to store the position of the cursor
as of the last redraw are renamed from vd_mdirty{x,y} to vd_mold{x,y}.
The associated comment is fixed too. Also, their value is now expressed
in pixels, not in character columns/row.

MFC after: 1 week