This adds specific marshallers for all of the locations where a generic
marshaller is being used. It also provides va_marshallers to reduce the
chances that we get stack traces from perf going through ffi_call_unix64.
This prevents some of our generic object implementation tests
from working with gesture objects. Instead, add g_return_if_fail
checks in all the gesture constructors.
This gesture implementation recognices swipes on any direction.
The "swipe" signal has the X/Y velocity vector components, so
those can be used for direction guessing and velocity thresholds.
