blurhash_transcoder: prevent out-of-bound reads with <8bpp images (#20388)

The Blurhash library used by Mastodon requires an input encoded as 24
bits raw RGB data. The conversion to raw RGB using Imagemagick did not
previously specify the desired bit depth. In some situations, this leads
Imagemagick to output in a pixel format using less bpp than expected.
This then manifested as segfaults of the Sidekiq process due to
out-of-bounds read, or potentially a (highly noisy) memory infoleak.

Fixes #19235.

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