Use signed instead of unsignet varint for ID and check for values out of varint range

git-svn-id: http://svn.osgeo.org/postgis/trunk@12199 b70326c6-7e19-0410-871a-916f4a2858ee
Remove flagging for first vertex and just substract with 0 instead

git-svn-id: http://svn.osgeo.org/postgis/trunk@12200 b70326c6-7e19-0410-871a-916f4a2858ee
First varint cunit test

git-svn-id: http://svn.osgeo.org/postgis/trunk@12201 b70326c6-7e19-0410-871a-916f4a2858ee