sin-ack a10ad24c76 LibGfx: Make JPGLoader iterate components deterministically ...

JPGLoader used to store component information in a HashTable, indexed
by the ID assigned by the JPEG file.  This was fine for most purposes,
however after f89e8fb7 this was revealed to be a flawed implementation
which causes non-deterministic iteration over components.

This issue was previously masked by a perfect storm of int_hash being
stable for the integer values 0, 1 and 2; and AK::HashTable having just
the right amount of buckets for the components to be ordered correctly
after being hashed with int_hash. However, after f89e8fb7,
malloc_good_size was used for determining the amount of space for
allocation; this caused the ordering of the components to change, and
images started showing up with the red and blue channels reversed. The
issue was finally determined to be inconsistent ordering after randomly
changing the order of the components caused Huffman decoding to fail.

This was the result of about 10 hours of hair-pulling and repeatedly
doing full rebuilds due to bisecting between commits that touched AK.
Gunnar, I like you, but please don't make me go through this again. :^)

Credits to Andrew Kaster, bgianf, CxByte and Gunnar for the debugging
help.

2021-05-31 17:26:11 +01:00

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