input_clock: add new function to handle buffering + test

Those functions are currently specific interpretation of the values that are already returned by input_clock_GetState(), ie. computed from system start, stream start, system buffering duration and stream buffering duration as well as the clock pause date or the current date.

The rationale for separating the computation in those functions instead of input_clock_GetState is that the latter exposes an internal state of the input_clock buffering code, which prevents modifying it in a different way -- like exposing multiple buffering segments for instance. Exposing the computations by "intent" instead of by "current state" enables abstracting away the buffering from the es_out.