A colorized image's color channel is the paint's samples repeated once a
pixel, and nothing bounded its size: an image colorized in Registration in
a document with many spot swatches could allocate without limit, and an
overflowing length panicked inside repeat. The length is now computed with
checked arithmetic and refused above MAX_RASTER_ALLOC, the image's own
decode budget, and the export leaves such an image out with the existing
"an image couldn't be decoded" warning instead of drawing it uncolored.