Creates the alpha plane: one component, the same width and height, at bitsPerComponent.
public bool TryCreateAlpha(int bitsPerComponent, bool premultiplied, scoped out DecodedImageBuilder alpha)
The alpha's bits per component, 1 to 16.
Whether the colour samples are premultiplied by it (SMaskInData 2).
The alpha builder, filled like this one and built with it.
false when an alpha plane exists already.ISO 32000-2 §7.4.9, §8.9.5.1 Table 87 (SMaskInData).