Use lengths 81 and 82 to signal extensions

[aaaaalbert]

Blurhashes start with a length field. Do the math and you shall see: x and y component counts from 1 to 9 can be encoded into values 0 to 80, occupying 81 of 83 possible values a single base-83 digit can take.

Encodings of 81 and 82 yield tall-and-narrow rectangular Blurhashes. I argue that it is more useful for these values to signal extensions to the default (3-channel sRGB DCT) Blurhash format. This enables experimentation while keeping the existing useful format intact.

For decoders to skip/error out on Blurhashes extended such, just inspect the length byte and only process lengths up to 80.

The "extension indicators" of 81 and 82 might

• be used for stable and experimental extensions, respectively
• be followed by another type or length digit
• (or variable lengths, see UTF-8's bag of tricks)

The functionality and semantics of the extension formats are subject to discussion: E.g., add alpha/transparency, do colorspace conversion like JPG, ...