gltfpack: Reuse time accessors between consecutive tracks

When tracks have custom time inputs (and resampling is disabled), we can
reuse inputs when they are equal between two tracks. This sharing is
only performed between consecutive tracks as it allows us to still
process tracks linearly.

gltf/write.cpp

@@ -1449,6 +1449,9 @@ void writeAnimation(std::string& json, std::vector<BufferView>& views, std::stri
 
 	size_t track_offset = 0;
 
+	size_t last_track_time_accr = 0;
+	const Track* last_track_time = NULL;
+
 	for (size_t j = 0; j < tracks.size(); ++j)
 	{
 		const Track& track = *tracks[j];
@@ -1458,7 +1461,17 @@ void writeAnimation(std::string& json, std::vector<BufferView>& views, std::stri
 
 		size_t track_time_accr = time_accr;
 		if (!track.time.empty())
-			track_time_accr = writeAnimationTime(views, json_accessors, accr_offset, track.time, settings);
+		{
+			// reuse time accessors between consecutive tracks if possible
+			if (last_track_time && track.time == last_track_time->time)
+				track_time_accr = last_track_time_accr;
+			else
+			{
+				track_time_accr = writeAnimationTime(views, json_accessors, accr_offset, track.time, settings);
+				last_track_time_accr = track_time_accr;
+				last_track_time = &track;
+			}
+		}
 
 		std::string scratch;
 		StreamFormat format = writeKeyframeStream(scratch, track.path, track.data, settings, track.interpolation == cgltf_interpolation_type_cubic_spline);