Update NURBS export logic so that knots are concatenated for all batched curves.

Added a test to cover batched NURBS export.

@@ -0,0 +1,492 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright 2022 Blender Foundation. All rights reserved. */

@@ -0,0 +126,4 @@
+    control_point_counts[i_curve] = tot_points;
+
+    /* For periodic linear curve, segment count = curveVertexCount.
+       For periodic cubic curve, segment count = curveVertexCount / vstep.

@@ -0,0 +175,4 @@
+  const int bezier_vstep = 3;
+  const OffsetIndices points_by_curve = geometry.points_by_curve();
+
+  for (int i_curve = 0; i_curve < geometry.curve_num; i_curve++) {

@@ -0,0 +177,4 @@
+
+  for (int i_curve = 0; i_curve < geometry.curve_num; i_curve++) {
+
+    const IndexRange curve_points = points_by_curve[i_curve];

@@ -0,0 +183,4 @@
+
+    const int start_verts_count = verts.size();
+
+    for (int i_point = start_point_index; i_point < last_point_index; i_point++) {

@@ -0,0 +294,4 @@
+
+    knots.resize(knots_num);
+    for (int i_knot = 0; i_knot < knots_num; i_knot++) {
+      knots[i_knot] = double(temp_knots[i_knot]);

@@ -0,0 +382,4 @@
+      });
+
+  if (number_of_curve_types > 1) {
+    WM_report(RPT_WARNING, "Cannot export mixed curve types in the same Curve object.");

@@ -0,0 +419,4 @@
+  }
+  else if (first_frame_curve_type != curve_type) {
+    WM_reportf(RPT_WARNING,
+               "USD does not support animating curve types. The curve type changes from %i to "

@@ -0,0 +52,4 @@
+void check_bezier_curve(const pxr::UsdPrim bezier_prim,
+                        const bool is_periodic,
+                        const int vertex_count);
+void check_nurbs_curve(const pxr::UsdPrim nurbs_prim,

@@ -0,0 +86,4 @@
+    return pxr::TfToken();
+  }
+
+  const size_t accumulatedControlPointCount = std::accumulate(

@@ -0,0 +387,4 @@
+      });
+
+  if (number_of_curve_types > 1) {
+    WM_report(RPT_WARNING, "Cannot export mixed curve types in the same Curves object.");

@@ -0,0 +427,4 @@
+  else if (first_frame_curve_type != curve_type) {
+    const char *first_frame_curve_type_name = nullptr;
+    RNA_enum_name_from_value(
+        rna_enum_curves_types, (int)first_frame_curve_type, &first_frame_curve_type_name);