Remove cairo dependency

We initially used Cairo for its bezier flattening algorithm. That
algorithm turned out to be a bit too imprecise at the scales we're
working at here (#17), so I ended up porting over some code from
Antigrain Graphics. The only other thing we used Cairo for was debug
output and coordinate transforms, so I just wrote the relevant vector
math in a small header file, deleted all debug output code and thus
eliminated the cairo dependency. This is a step towards Windows builds.

svg-flatten/src/main.cpp

@@ -422,7 +422,7 @@ int main(int argc, char **argv) {
     SVGDocument doc;
     cerr << "Loading temporary file " << frob << endl;
     ifstream load_f(frob);
-    if (!doc.load(load_f, "/tmp/debug.svg")) {
+    if (!doc.load(load_f)) {
         cerr <<  "Error loading input file \"" << in_f_name << "\", exiting." << endl;
         return EXIT_FAILURE;
     }