There were situations where the cursor was not erased/redrawn or its
position was marked as dirty even though it's not displayed. The code is
now more straightforward.

At the same, add a function to determine if the cursor covers a given
area. This is used by backends to know if they need to draw the cursor.

This new function should be paired with a new state in struct vt_device,
called vd_mshown, which indicates if the cursor should be displayed.
This again simplifies vd_bitblt_text_t callback's API.

MFC after:	1 week