[rttopo-dev] Tolerance 0 support in master branch

[Sandro Santilli]

I've changed the semantic of tolerance parameter value 0
from "automatic" to "zero", and added support for value -1
to mean "automatic".

This basically adds support for really specifying 0 as
the tolerance value, which would result in no snapping
ever occurring.

With this change, I tried the spatialite testsuite (check_topology2d)
and got a failure:

 rtt_AddIsoNode failed TopoGeo_AddLineString() #1 error: rtt_AddIsoNode failed

The failure isn't there if I revert the librttopo change.

Alessandro: could you help reducing the testcase to figure out
what's going on ? Spatialite is the only existing testsuite
for librttopo...

--strk;