bug#15783: 24.3; posn-at-x-y is relative to the buffer area, posn-at-point is relative to the text area.

[椎野 裕樹]

On Emacs 24 with non-nil header-line-format, y offset returned by
`posn-at-point' is relative to the text area *not including the header
line*.  However, `posn-at-x-y' takes y offset relative to the buffer
area *including the header line*.

Is this by design?

When I get the position by `posn-at-point', and then convert the
position to the point by `posn-at-x-y', the resulting point does not
match the point given to `posn-at-point'.

This seems to me a serious inconsistency.

On Emacs 22 and 23, both functions are relative to the buffer area
including the header line.  It's fine.

Cheers,
Yuki Shiino