[cairo-output-stream] Write out floats to 18 digits after decimal point

We write floats using %f as the scientific format used by smarter %g is
invalid in PS/PDF.  %f however by default rounds to five digits after
decimal point.  This was causing precision loss and making the newly
added degenerate-pen test fail for PDF.  We now print up to 18 digits
which is as many bits doubles can accomodate.  We can be smarter, but
that's for another commit.

src/cairo-output-stream.c

@@ -236,7 +236,7 @@ _cairo_dtostr (char *buffer, size_t size, double d)
     if (d == 0.0)
 	d = 0.0;
 
-    snprintf (buffer, size, "%f", d);
+    snprintf (buffer, size, "%.18f", d);
 
     locale_data = localeconv ();
     decimal_point = locale_data->decimal_point;