An image whose pixels can't be decoded, such as a PNG cut short by an
interrupted download, drew nothing on the canvas and the status bar
said nothing. Every render decoded it again, so each frame that drew it
paid for a decode that fails.
The renderer now notes such an image by its content and does not decode
it again. Each render still counts it in decode_failures, so an export
reports it as before. An image refused only because the images being
drawn leave it no room within twice the decoding allowance is not
noted, and the next render tries it again.
On the canvas the image draws as a light gray box crossed from corner
to corner, and the status bar says once for each image that it couldn't
be decoded. Exports still draw nothing there.
Docs: docs/target-app-parity.md.
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