[unity] Fixed too high Mesh vertex count when multiple submeshes are used (in combination with clipping). Supported on Unity 2019.3+. Closes #1894.

spine-unity/Assets/Spine/Runtime/spine-unity/Mesh Generation/MeshGenerator.cs

@@ -27,6 +27,10 @@
  * THE SPINE RUNTIMES, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *****************************************************************************/
 
+#if UNITY_2019_3_OR_NEWER
+#define MESH_SET_TRIANGLES_PROVIDES_LENGTH_PARAM
+#endif
+
 // Not for optimization. Do not disable.
 #define SPINE_TRIANGLECHECK // Avoid calling SetTriangles at the cost of checking for mesh differences (vertex counts, memberwise attachment list compare) every frame.
 //#define SPINE_DEBUG
@@ -1117,7 +1121,11 @@ namespace Spine.Unity {
 			mesh.subMeshCount = submeshCount;
 
 			for (int i = 0; i < submeshCount; i++)
+#if MESH_SET_TRIANGLES_PROVIDES_LENGTH_PARAM
+				mesh.SetTriangles(submeshesItems[i].Items, 0, submeshesItems[i].Count, i, false);
+#else
 				mesh.SetTriangles(submeshesItems[i].Items, i, false);
+#endif
 		}
 		#endregion