To be able to cancel an operation, an application should register a
callback function that returns a non-zero value whenever the active
operaton should be cancelled. A backend can invoke this callback function
to query the application for a pending cancellation request.
A helper function is added to simplify implementing the devic_dump()
function on top of the device_read() function, and enable progress
events automatically.
Using a resizable memory buffer allows to allocate the right amount of
memory inside the backend, avoiding having to know the required buffer
size in advance.
The logbook ringbuffer is now considered empty if any of the pointers is
outside the valid ringbuffer area. Compared to checking only against a
special empty value, this approach makes the code more robust against
invalid pointers.
Although calibration is optional, it's highly recommended because it
reduces the transfer time considerably. The calibrate function is
removed from the public api.
Introducing a common base class allows to share more code between the
backends. Sharing the fingerprint data eliminates the need to pass it
with a function parameter.
