bug#4094: 23.1.50; posn-at-point gets upset by overlays with 'invisible and 'display

[Lennart Borgman]

Some more thoughts: There seem to be several small inconsistencies
around here. I guess the intended behaviour is

For invisible overlays (and dito buffer text, but it is overlays I
have been looking at now):

- The "stop" point on the display string property should be the first
character. That seems to be the case now, but often the first char is
missed/jumped over, see below.

- When calling forward-char or backward char they should stop there.
Forward char often does that, but not always. Backward char does not
seem to do that.

- Similar problem with vertical-motion. It often misses the first char
and goes to the column after the display string instead.