gltfpack: Refactor constant track support

Instead of determining whether the track is constant when writing it, we
now have an explicit bit per track that says if it is. We still validate
the track length to make sure time or pose input is going to match the
output in terms of keyframe count.

gltf/animation.cpp

@@ -299,6 +299,7 @@ void processAnimation(Animation& animation, const Settings& settings)
 		if (isTrackEqual(track.data, track.path, frames, &track.data[0], track.components))
 		{
 			// track is constant (equal to first keyframe), we only need the first keyframe
+			track.constant = true;
 			track.data.resize(track.components);
 
 			// track.dummy is true iff track redundantly sets up the value to be equal to default node transform

gltf/gltfpack.h

@@ -61,6 +61,7 @@ struct Track
 	cgltf_node* node;
 	cgltf_animation_path_type path;
 
+	bool constant;
 	bool dummy;
 
 	size_t components; // 1 unless path is cgltf_animation_path_type_weights

gltf/write.cpp

@@ -1225,10 +1225,10 @@ void writeAnimation(std::string& json, std::vector<BufferView>& views, std::stri
 	{
 		const Track& track = *tracks[j];
 
-		bool tc = track.data.size() == track.components;
+		assert(track.data.size() == track.components * (track.constant ? 1 : animation.frames));
 
-		needs_time = needs_time || !tc;
-		needs_pose = needs_pose || tc;
+		needs_time = needs_time || !track.constant;
+		needs_pose = needs_pose || track.constant;
 	}
 
 	size_t time_accr = needs_time ? writeAnimationTime(views, json_accessors, accr_offset, animation.start, animation.frames, settings) : 0;
@@ -1243,8 +1243,6 @@ void writeAnimation(std::string& json, std::vector<BufferView>& views, std::stri
 	{
 		const Track& track = *tracks[j];
 
-		bool tc = track.data.size() == track.components;
-
 		std::string scratch;
 		StreamFormat format = writeKeyframeStream(scratch, track.path, track.data, settings);
 		BufferView::Compression compression = settings.compress && track.path != cgltf_animation_path_type_weights ? BufferView::Compression_Attribute : BufferView::Compression_None;
@@ -1260,7 +1258,7 @@ void writeAnimation(std::string& json, std::vector<BufferView>& views, std::stri
 
 		comma(json_samplers);
 		append(json_samplers, "{\"input\":");
-		append(json_samplers, tc ? pose_accr : time_accr);
+		append(json_samplers, track.constant ? pose_accr : time_accr);
 		append(json_samplers, ",\"output\":");
 		append(json_samplers, data_accr);
 		append(json_samplers, "}");