Add an alpha channel. The newly created alpha channel will have constant opaque alpha, except in the special case of TGrayscaleImage.TreatAsAlpha = True (where the contents will be copied to alpha, and intensity set to white).
Should we treat grayscale image as pure alpha channel (without any color information) when using this as a texture.
This property is meaningful only for a small subset of operations, right now: only when creating OpenGL texture from this image. If True, then the grayscale pixel data will be loaded as alpha channel contents (GL_ALPHA texture for OpenGL, it modifies only the fragments alpha value, it doesn't have any "color" in the normal sense). It is also the only way for TGrayscaleImage to return AlphaChannel <> acNone.