toArray method
dart array can not expand default so have to set array length enough first.
Implementation
// It's working, but ugly. consider to adds a new function:
// toBufferAttribute(BufferAttribute attribute, int index) ???
toArray([array, int offset = 0]) {
array ??= List<double>.filled(3, 0.0);
array[offset] = r;
array[offset + 1] = g;
array[offset + 2] = b;
return array;
}