Changed parameter types from const to non-const reference, to prevent dangling references to temporary variables.

git-svn-id: svn://svn.code.sf.net/p/agg/svn/agg-2.4@94 ab9de31f-a152-4fe9-8809-95f286f9ca89
Backport 2.5 changes except X11

Signed-off-by: Timothy Gu <timothygu99@gmail.com>