We previously handed it a wrapper that was supposedly type-safe but
really internally cast the buffer to bytes. That was rather pointless as
the `_bytes` method could always be used instead. We replace the
semantics by a utility which allocates an appropriate buffer with the
internal limits, wrapping the exact sequence identified previously in
the example as a pattern.
By adding the type interpretation to `BufferLayoutPreference` (simply
computed from the same fields that are trivial to fetch in
`ReadoutLayout` with the rest) we can allocate an appropriate
`DecoderResult` without any `Decoder` instance. We still want a Limits
instance though. This lets a caller read _typed_ data from the `_bytes`
methods which perform less allocations. It also lets them read multiple
planes in a more convenient interface. See, `read_image` will keep its
planar treatment bug for a little bit so you don't accidentally break
during an upgrade.
This also implies less code is being monomorphized, just a bonus.
Lilith River