Greeter gets stuck on random input

[Michael Zanetti]

multi touch taps can break the greeter. Here's one way to repro:

* With one finger, start dragging the greeter a bit, don't release
* place a second finger on the greeter
* release first finger => greeter slides back to locked state
* put the first finger on the screen again => greeter jumps to finger again
* release finger, then release second finger

=> greeter stuck half way across the screen

[Michael Terry (mterry)]

I can reproduce on krillin/rtm, but not on mako/vivid. So closing for vivid.