*alpha* parameter handling in _ImageBase._make_image

This PR expands and clarifies code logic in _ImageBase._make_image

related to the *alpha* parameter, and ensures that `~.Axes.imshow`

exhibits the following behavior:

1. When the image does not have alpha channel:

- *alpha* not specified: default to 1.0.

- scalar *alpha*: broadcast.

- array *alpha*: concatenate.

2. When the image has an alpha channel:

- *alpha* not specified: nothings needs to be done.

- scalar *alpha*: multiply the alpha channel with user-specified scalar.

- array *alpha*: replace the alpha channel with user-specified array.