code: func MakeVertexSlice(shader *Shader, len, cap int) *VertexSlice {
  	if len > cap {
  		panic("failed to make vertex slice: len > cap")
  	}
  	return &VertexSlice{
  		va: newVertexArray(shader, cap),
  		i:  0,
  		j:  len,
  	}
  }
docs: // MakeVertexSlice allocates a new vertex array with specified capacity and returns a VertexSlice
  // that points to it's first len elements.
  //
  // Note, that a vertex array is specialized for a specific shader and can't be used with another
  // shader.