Previously all scrolling relied on "clicking" the up/down or left/right scroll buttons
which made it unprecise and to always scroll at the same speed.
Now we pass the scroll delta directly to the xorg input driver so the scroll is more responsinve and adaptive.
This allows Xvnc to build with xorg-server 1.20.7, which requires OS
layers to implement a ddxInputThreadInit function when configured with
--enable-input-thread (the default).
relevant xorg-server commit: e3f26605d85d987da434640f52646d728f1fe919