Hard cropping (performance optimization)

[Cre]

Allow cropping or masking a layer so that it actually increases performance when the mask is active.

This "hard cropping" would (temporarily) discard everything (all input data) outside the given rectangle, and not just mask the output area/rectangle like the regular masking in Affinity does.

Also, layer FX's would see the area smaller,  and there might be some differences with blur etc. but it would still be a valuable tool to increase the performance on a large document, by focusing the computing power to the small ("hard cropped") area at a time.