mouse: workaround for too fast vertical scroll

[metalefty]

Provided by @seflerZ at #150.

I tested the patch, 10ms interval looks enough.

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To enable this hack, edit sesman.ini like this:

[SessionVariables]
(some existing lines)
XRDP_XORG_TOUCHPAD_SCROLL_HACK=yes

[matt335672]

That looks fine to me.

I'm happy to go with 10ms. If it turns out to need tweaking later, we can get the environment variable to take a numeric argument as an option, but at the moment there's no need to over-complicate it.