In TT2, keyboard/gamepad navigation is implemented by moving the mouse
cursor using IZ command. It did not work in browsers where mouse cursor
could not be moved or when mouse movement was disabled from the menu.
Now IZ command updates the internal mouse coordinates even when the
cursor cannot be moved.
This replaces v_str* functions with new svar_* functions.
The new svar_* functions use 1-based string variable index, so that
callers don't have to decrement string numbers from SCO.
kichikuou