Provide API to query colorspace/chroma before decoding

[farindk]

We should have a function that allows to query the internal colorspace/chroma of an image before we actually decode this. This might be useful to specify the desired output colorspace depending on the coded image (e.g. RGB or monochrome output). See #636. This is complicated by the fact that the image may have transformations applied are is a derived image (overlay) what may require a change in the internal colorspace.