vad : revisit timestamp alignment/mapping (#3173)

* vad : revisit timestamp alignment/mapping

This commit improving the timestamp alignment by introducing a mapping
table, adding intermediate reference points for longer segments, and
binary search for lookups.

The motivation for this changes is to address issues with the currently
solution where zero-length segments are possible, and also to improve
the precision of the VAD timestamps.

Refs: #3162

* vad : use uint64_t for time mapping

This commit changes the type of the `processed_time` and `original_time`
fields in the `vad_time_mapping` struct from `double` to `uint64_t`.

The motivation for this change is made to improve precision and avoid
floating-point inaccuracies and also be consistent with other part of
the code base that use `uint64_t` for time representation.

This is a part of a refactoring where I'm also going to change the
vad_segment_info struct to use `uint64_t` for the start and end times.
This is the reason for the not so pleasant conversion and casts in the
code at the moment.

* vad : change vad_segment_info and whisper_vad_segment to use uint64_t

* vad : use int64_t instead of uint64_t for timestamps

To be consistent with other timestamps in the codebase.

* vad : add centisecond conversion functions

* vad : extract vad processing from whisper_full_with_state

This commit extracts the VAD processing from the
`whisper_full_with_state` function into the `whisper_full` and
`whisper_full_parallel` functions.

The motivation for this is that I did not take into account that when
`whisper_full_parallel` is called with `n_processors > 1`, then the
vad processing would not be applied correctly. Instead the VAD
processing should be done prior to processing in the case of
`whisper_full_parallel`.

* vad : remove filtered_n_samples from whisper_vad

The commit removes the parameter `filtered_n_samples` from the
`whisper_vad` function signature and its usage, as it is no longer
needed since filtered samples is now a vector (previously it was a
float*)

The motivation for this is to simplify the usage of this function.

* vad : remove vad_mapping_table_initialized flag

* vad : fix leaning (none) of pointer/references

Author: danbev