Added Source:getDuration (thanks Boolsheet!)

For streaming sources, the duration may not always be sample-accurate and may return -1 if it cannot be determined at all.

src/modules/audio/wrap_Source.h

@@ -42,6 +42,7 @@ int w_Source_setVolume(lua_State *L);
 int w_Source_getVolume(lua_State *L);
 int w_Source_seek(lua_State *L);
 int w_Source_tell(lua_State *L);
+int w_Source_getDuration(lua_State *L);
 int w_Source_setPosition(lua_State *L);
 int w_Source_getPosition(lua_State *L);
 int w_Source_setVelocity(lua_State *L);