Fix truncation of Atkinson and Floyd-Steinberg dithering routines in dithering.js

[MatrixNeoKozak]

The 'atkinson' and 'bwr_floydsteinberg' branches in ditheringCanvasByPalette are truncated mid-function, causing syntax errors and incomplete pixel value updates. The fix completes the functions to properly apply error diffusion. Also corrects a typo in canvas2bytes where red/green/blue indices were swapped (b, g, b instead of r, g, b), which caused incorrect bit detection.

[javl]

Please don't create ai generated pull requests without checking the code.

1. The change to dithering doesn't matter: that entire canvas2bytes() is never even called (I should just take it out)
2. The second edit (rbg > rgb order) does seem to make sense, I'll look into it
3. The last part contains errors (ifErr seems to be some sort of copy paste error?), breaking the entire script