`alpha` parameter handling in `_ImageBase::_make_image`

This PR expands and clarifies code logic in `_ImageBase::_make_image` related to

the `alpha` parameter, and ensure 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.