Cache whether an update performs a shift, and expose a method to determi...

Cache whether an update performs a shift, and expose a method to determine after an update if a shift occurred.

We needed a method to reliably determine if a cube shift occurred, and while there is a `checkForShift` method in `CyclicalBuffer`, the call in `KinfuTracker::operator()` depends on arguments constructed in the method immediately before. To get around this, we can simply cache the result of `CyclicalBuffer::checkForShift` in `operator()`, and expose a method to return it.
Typically we `inline` and `const`-qualify getter functions.