improve throughput of DouglasPeuckerFilter 

don't create copy of shape/line if no point was removed

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3260 25d90789-57f7-4ee0-8453-03a3dfeeeb22
simplify code, no functional change

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3261 25d90789-57f7-4ee0-8453-03a3dfeeeb22
small performance improvements: use compiled tags in a few more places

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3262 25d90789-57f7-4ee0-8453-03a3dfeeeb22