add vte_terminal_get_scroll_on_output

[t-bltg]

Hi,

I've tried to use mlterm as a libvte engine as advertised in https://github.com/arakiken/mlterm/blob/master/gtk/README.

It works using gnome-terminal but fails on e.g. xfce4-terminal:

$ LD_PRELOAD=$MLTERM_PREFIX/lib/libvte-2.91.so gnome-terminal
$ LD_PRELOAD=$MLTERM_PREFIX/lib/libvte-2.91.so xfce4-terminal
xfce4-terminal: symbol lookup error: xfce4-terminal: undefined symbol: vte_terminal_get_scroll_on_output

Here is a patch defining vte_terminal_get_scroll_on_output (implementation to be discussed maybe: I don't know much of the mlterm internals...).

[arakiken]

Thanks you! Sorry for my late reply.
I merged your patch with a little modification.

[t-bltg]

Awesome, and many thanks for the hard work in mlterm !