gltfpack: Enable pruning during simplification

This should probably be unambiguously a good idea, especially given the
default low error target. If this ends up being a bad idea, we can
revert this or expose a separate option; for now, -slb will disable this
because pruning doesn't respect locked borders.

gltf/mesh.cpp

@@ -625,9 +625,13 @@ static void simplifyMesh(Mesh& mesh, float threshold, float error, bool attribut
 	size_t target_index_count = size_t(double(mesh.indices.size() / 3) * threshold) * 3;
 	float target_error = error;
 	float target_error_aggressive = 1e-1f;
+
 	unsigned int options = 0;
 	if (lock_borders)
 		options |= meshopt_SimplifyLockBorder;
+	else
+		options |= meshopt_SimplifyPrune;
+
 	if (debug)
 		options |= meshopt_SimplifyInternalDebug;