Override get_invalidated_by_change() with the same logic as
get_required_for_output(), to avoid unnecessary invalidation.
Avoid format conversion when input and aux have the same format.
Add pass-through fast path when the ROI is completely inside/
outside the cropped rectangle.
Use bulk memcpy(), instead of per-pixel test-and-copy, in
process().
... layers with alpha channel
add an operation that selectively outputs aux
or source if out/inside the rectangle (x,y,width,height),
independently of the alpha channel and use it
in filters split preview.
mostly copied from gimp:mask-components
