map-applicable-translators: refactor the code
map-applicable-translators: refactor the code

We make map-over-presentations-containing-position a local function in
map-applicable-translators. That leads to a small simplification of
the function body.

Small improvement is handling situations where x/y are NIL, then we do
not perform position test. That will be useful for transformation
outside the window in memory.

slyrus/mcclim,slyrus/mcclim