change encoding format, ~7% smaller files on kodim test suite

[kmar]

by changing the encoding a bit, I've managed to reduce qoi file sizes on the kodim testsuite
the new encoding operates in two modes, color (default) and alpha.
alpha mode is more suited for images with varying alpha channel, improving the compression ratio even more
improving the hashing function may improve the sizes even further, but I kept it as is to minimize performance impact

[aligokalppeker]

This PR has made this efficient and simple image format far more complex. I rather choose the original version. Performance impact should be considered (measured) for this case. If we desire more efficient compression over performance, there is many other formats out there.

[kmar]

This PR has made this efficient and simple image format far more complex. I rather choose the original version. Performance impact should be considered (measured) for this case. If we desire more efficient compression over performance, there is many other formats out there.

your definition of "far more complex" seems rather odd, the useless 16-bit RLE mode is gone and alpha mode tries to improve a problem with complex alpha channels where the original encoding just struggles. I prefer a better compression ratio with very little cost. Also when this encoding is combined with 16x16 blocks here #33, the ratio is improved further 7% on the kodim suite. That's significant - ask anyone who actually ever did program a compressor.
7% is roughly what you get with optimal LZ parsing, which is way more demanding.
Anyway - since there's apparently no will to accept encoding improvements, I'm closing this PR. What a waste of time.

[aligokalppeker]

By the complexity, i mean about code complexity. As i see in the change set, it is not simple additions/modifications you made here, it is rather change in many different points in the block.

[aligokalppeker]

Sorry, but I do not want to make you close this PR, but only discuss and improve its results