MikktspaceTangentGenerator should ignore meshes without triangles

[stephengold]

The experimental tangent generator in jme3-core assumes that all meshes are composed of triangles. If you feed it a mesh whose mode is Mesh.Mode.Points or Mesh.Mode.Lines, it will usually crash with a BufferUnderflowException:

    java.nio.BufferUnderflowException
        at java.base/java.nio.Buffer.nextGetIndex(Buffer.java:699)
        at java.base/java.nio.DirectFloatBufferU.get(DirectFloatBufferU.java:324)
        at com.jme3.util.mikktspace.MikkTSpaceImpl.getPosition(MikkTSpaceImpl.java:77)
        at com.jme3.util.mikktspace.MikktspaceTangentGenerator.getPosition(MikktspaceTangentGenerator.java:719)
        at com.jme3.util.mikktspace.MikktspaceTangentGenerator.generateSharedVerticesIndexList(MikktspaceTangentGenerator.java:286)
        at com.jme3.util.mikktspace.MikktspaceTangentGenerator.genTangSpace(MikktspaceTangentGenerator.java:164)
        at com.jme3.util.mikktspace.MikktspaceTangentGenerator.genTangSpaceDefault(MikktspaceTangentGenerator.java:126)
        at com.jme3.util.mikktspace.MikktspaceTangentGenerator.generate(MikktspaceTangentGenerator.java:118)
  

A mesh composed entirely of lines or points doesn't need tangents. Since MikktspaceTangentGenerator processes models containing multiple geometries, it should quietly skip any geometries without triangles.