Process Mapnik: fix request for canvas properties

as _check-function is no longer exposed to pgsql, we need to query
_match-function (or use another method, but this is the easiest).
we use a dirty hack - when render_context.bbox is null, the type of
object instead of style-element is passed.
plpython: re-implement member of selectors (relations)
plpython: member of links: way -> node
plpython: compile link selector: check parent and link tags
plpython: Make sure, that all link tags are strings
osm2pgsql: objects_by_id() generates objects from list input
plpython: re-implement members link selector
Move eval functions into scope of 'match' function
db.osm2pgsql.objects() requires bbox as first parameter
Parse Condition: bugfix, values with unit (e.g. [distance<50m])
plpython: re-implement near link selector

- also, objects() accepts additional parameters: add_columns,
  add_param_type, and add_param_value
plpython: remove obsolete code
plpython: remove more obsolete code (pgmapcss_render_context)
add profiling information
plpython: eval: text_transform() (also property text-transform)