Add support for alpha channel in st.image's channels argument

[SebastianScherer88]

Checklist

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Summary

The current implementation of the st.image image display utility only supports two options in its channels argument: RGB and RBG. It would be great if the displaying of images with an additional fourth alpha channel (controlling the opacity) would be supported, maybe via additional supported channel modes RGBA and RBGA.

Why?

Rendering images with alpha channel masks have the masks ignored, and so default to the image color channel values in the areas where the mask indicates transparency.

How?

See summary section - hopefully that is feasible/makes sense.

As an alternative, I will try dynamically rendering images with masks to take on the colour of currently selected background colours, although that is finicky.

Additional Context

example where the transparent section of the image is still shown despite the mask being 0 in those pixels:

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