3D viewer: move ray based picking into own function and use color map as fallback
3D viewer: move ray based picking into own function and use color map as fallback

If now an object should be picked, a ray is cast first. If this results
in a node being picked, it is used. Otherwise, the color based picking
function is used. The latter will find a result for sure, but takes
maybe a second longer.

See acardona/CATMAID#890

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