enable partial transparency for output image when using Colormap

[vincentsarago]

In the past (before rio-tiler 5.0, https://cogeotiff.github.io/rio-tiler/migrations/v5_migration/#per_dataset-per_band-mask) we allowed mask values between 0 and 255 (cogeotiff/rio-tiler#580 (comment)).

We removed this feature because we switched to numpy MaskedArray (either 0 or 255), which enabled per-band mask and much more.

If people needs to work with non-premultiplied alpha bands then please open a ticket in rio-tiler and we can discuss ways to go forward.

At the same time, it came twice in a week that people were asking for non-premultiplied alpha bands in colormap, and this is somehow maybe easier to fix