On touchpads gtk_gesture_get_sequences() was called without a
corresponding g_list_free(). The same was true for touchscreens
if due to some reason only a single sequence was found.
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.
