Sub-tile light sources about libfov HOT 1 OPEN

In order to jitter the origin of a light (say, if there's a flickering
fire), the finest grain you can have is moving the origin by a whole tile
in some direction. Doing that causes some ugly artifacts, especially when
there is a flickering light right next to an opaque cell:

frame 1, origin at the centre of 'f'
.....
.....
....
...f#
....#
....#
...#

frame 2, origin one tile north of 'f'
.....
.......
.......
.......
...f###
....#
...#

When the animation flicks quickly between these two frames (and the effect
is even more noticeable with a larger radius), a rather jarring effect is
produced.

If you could specify the origin of the light with sub-tile granularity,
this problem could be avoided.