[Bug] Stacking affines/grid transforms with different keys

[mibaumgartner]

Description
When stacking affine/grid transforms with different keys per transform, the whole transformation will be applied to all keys.

Proposal
Temporarily, check keys when stacking transforms and raise error if keys are changing. Additionally, open a feature request issue to support this special case (I do not think we should add support for this right away because it will make things fairly complicated)