scroll encoding can be costly - use it more parsimoniously

[totaam]

In particular, the html5 client has to use a secondary buffer to memcopy the current pixel areas without causing visual corruption. (perhaps this can be fixed?)

We can keep track of the last event time for:

• successfully using scroll encoding
• processing a mouse wheel event

And only trigger the scroll detection when it is more likely to succeed.
Also let the clients specify how aggressive this heuristic should be.