std::chrono::system_time on osx isn't in nanoseconds (#2795)

* Allow the scan parameters to be changed in a detector.

* Fix warning.

* Fix for clang on osx which std::chrono::system_time has a duration in microseconds not nanoseconds. ^

* Revert "Allow the scan parameters to be changed in a detector."

This reverts commit 2a5760aacf.

* Fix second instance.

* Static not needed when inline.

---------

Co-authored-by: Charles Galambos <charles@reactai.com>
Co-authored-by: Davis E. King <davis685@gmail.com>

dlib/dnn/utilities.h

@@ -73,7 +73,7 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    static size_t tensor_index(
+    inline size_t tensor_index(
         const tensor& t,
         const long sample,
         const long k,

dlib/media/ffmpeg_demuxer.h

@@ -876,7 +876,7 @@ namespace dlib
                     frame decoded;
                     frame src;
                     src.f           = std::move(avframe); //make sure you move it back when you're done
-                    src.timestamp   = system_clock::time_point{nanoseconds{timestamp_ns}};
+                    src.timestamp   = system_clock::time_point{duration_cast<system_clock::duration>(nanoseconds{timestamp_ns})};
 
                     if (src.is_image())
                     {

dlib/media/ffmpeg_utils.h

@@ -818,9 +818,9 @@ namespace dlib
 
                     dst.f           = std::move(tmp);
                     dst.f->pts      = tracked_samples;
-                    dst.timestamp   = system_clock::time_point{nanoseconds{av_rescale_q(tracked_samples,
+                    dst.timestamp   = system_clock::time_point{system_clock::duration{av_rescale_q(tracked_samples,
                                                                                         {1, dst_sample_rate},
-                                                                                        {nanoseconds::period::num, nanoseconds::period::den})}};
+                                                                                        {system_clock::duration::period::num, system_clock::duration::period::den})}};
                     tracked_samples += dst.nsamples();
 
                 }
@@ -907,10 +907,10 @@ namespace dlib
 
                     while (av_audio_fifo_size(fifo.get()) >= frame_size)
                     {
-                        const system_clock::time_point timestamp{nanoseconds{av_rescale_q(
+                        const system_clock::time_point timestamp{system_clock::duration{av_rescale_q(
                                 sample_count,
                                 {1, in.sample_rate()},
-                                {nanoseconds::period::num, nanoseconds::period::den})}};
+                                {system_clock::duration::period::num, system_clock::duration::period::den})}};
 
                         frame out(in.sample_rate(), frame_size, in.layout(), in.samplefmt(), timestamp);