Persist color channels in `rasterize()`

[clwgg]

Fixes scverse/spatialdata-plot#249

Please see the above spatialdata-plot issue for a problem description.

This is just a draft of how this issue could be addressed. I could see several different routes this could be tackled, including:

• splitting the tests into a separate function
• using if schema in (Image2DModel, Image3DModel): logic instead of the current try: except:
• using get_channels() instead of xdata.coords["c"].values, although this is a bit awkward since xdata is not the correct type for get_channels(), so one would have to either modify get_channels() or call it on data instead of xdata which feels a little less clean

Let me know what you think!