Can't rasterise a Compound Mask Layer to merge together different Mask Layers inside it

[Frozen Death Knight]

There is currently no easy way to rasterise a Compound Mask Layer, which would make it easier to use this feature for merging various Mask Layers.

This is also important because exporting to PSD will break Compound Mask Layers, since that feature doesn't exist in Photoshop. Being able to simplify the merging process as well as automatically rasterise these layer types during export would make Affinity more compatible with Photoshop for those who still rely on its file format.