gltfpack: Change the defaults to copy textures to output

To fix consistency issues with output by default, we now default to
copying (.gltf)/embedding (.glb) textures with or without compression.

There are cases when the previous behavior is desireable. This change
adds a new option, -tr, that keeps the references to original textures
without processing. Notably, this also works for .glb outputs, allowing
to avoid embedding textures into .glb.

When compressing textures, -tr is not currently supported because we do
need to produce new file data. In the future we could consider
supporting -tr by saving the .ktx2 files side by side with the original
images.

gltf/gltfpack.cpp

@@ -1348,9 +1348,9 @@ int main(int argc, char** argv)
 		{
 			settings.texture_flipy = true;
 		}
-		else if (strcmp(arg, "-tcp") == 0)
+		else if (strcmp(arg, "-tr") == 0)
 		{
-			settings.texture_copy = true;
+			settings.texture_ref = true;
 		}
 		else if (strcmp(arg, "-tj") == 0 && i + 1 < argc && isdigit(argv[i + 1][0]))
 		{
@@ -1459,6 +1459,7 @@ int main(int argc, char** argv)
 			fprintf(stderr, "\t-tp: resize textures to nearest power of 2 to conform to WebGL1 restrictions\n");
 			fprintf(stderr, "\t-tfy: flip textures along Y axis during BasisU supercompression\n");
 			fprintf(stderr, "\t-tj N: use N threads when compressing textures\n");
+			fprintf(stderr, "\t-tr: keep referring to original texture paths instead of copying/embedding images\n");
 			fprintf(stderr, "\tTexture classes:\n");
 			fprintf(stderr, "\t-tc C: use ETC1S when encoding textures of class C\n");
 			fprintf(stderr, "\t-tu C: use UASTC when encoding textures of class C\n");
@@ -1543,6 +1544,12 @@ int main(int argc, char** argv)
 		return 1;
 	}
 
+	if (settings.texture_ref && settings.texture_ktx2)
+	{
+		fprintf(stderr, "Option -tr currently can not be used together with -tc\n");
+		return 1;
+	}
+
 	if (settings.fallback && settings.compressmore)
 	{
 		fprintf(stderr, "Option -cf can not be used together with -cc\n");

gltf/gltfpack.h

@@ -141,7 +141,7 @@ struct Settings
 
 	bool texture_ktx2;
 	bool texture_embed;
-	bool texture_copy;
+	bool texture_ref;
 
 	bool texture_pow2;
 	bool texture_flipy;

gltf/write.cpp

@@ -910,7 +910,7 @@ void writeImage(std::string& json, std::vector<BufferView>& views, const cgltf_i
 {
 	bool dataUri = image.uri && strncmp(image.uri, "data:", 5) == 0;
 
-	if (image.uri && !dataUri && !settings.texture_embed && !settings.texture_copy)
+	if (image.uri && !dataUri && settings.texture_ref)
 	{
 		// fast-path: we don't need to read the image to memory
 		append(json, "\"uri\":\"");