Updated vdb_view to use GLFW3

This is an update for the vdb_view to work with the GLFW3. GLFW3 is not compatible with GLFW2, but may soon be the default in many systems (mine already is - Gentoo 64bits).

The update only changes the way the window is handled and how the callbacks are defined/implemented, everything else remains untouched.

Ps.: Sharing because it my be useful for other peoples, like: http://www.openvdb.org/forum/?place=msg%2Fopenvdb-forum%2FZSsEkkqFxXw%2F2hMf9okceCEJ