Reduce overhead of ST_Simplify calls against points and multipoints

It takes half the time to run against 800k point records now
(still double the time it takes to return those 800k records directly)

See #2745

git-svn-id: http://svn.osgeo.org/postgis/trunk@12577 b70326c6-7e19-0410-871a-916f4a2858ee