PNG out-of-range palette indices

[randy408]

I haven't looked too much into it but my initial tests with spngt on the two images at w3c/PNG-spec#96 appear to not decode with wuffs, probably because the out-of-range values are handled as an error, might want to change it to match libpng's defaults.

[nigeltao]

appear to not decode with wuffs

Can you share the code that leads to this conclusion?

wuffs/std/png/decode_png.wuffs

Lines 863 to 870 in a55e0a0

	// Set the remaining palette entries to opaque black.
	while i < 256 {
	this.src_palette[(4 * i) + 0] = 0x00
	this.src_palette[(4 * i) + 1] = 0x00
	this.src_palette[(4 * i) + 2] = 0x00
	this.src_palette[(4 * i) + 3] = 0xFF
	i += 1
	} endwhile

explicitly fills out-of-range palette entries to opaque black.

After running ./build-example.sh example/imageviewer from the Wuffs directory, the gen/bin/example-imageviewer X11 GUI program (which exercises Wuffs' PNG decoder) seems to agree with the w3c/PNG-spec#96 original post: gray border, opaque black top and (opaque blue | transparent) bottom. Use the comma key, in gen/bin/example-imageviewer, to cycle the background color (which shows through transparent pixels).