This helps with unifying how image data from IDAT and fdAT is handled.
This unification is a prerequisite for the follow-up commit where we
will avoid temporarily copying compressed image data into
`current_chunk.raw_bytes`.

…bytes`.

Before these changes we would store all image data in
`ChunkState::raw_bytes` before attempting to decompress them via
`ZlibStream`.  This is undesirable for performance, because 1) it copies
data unnecessarily into and out of `raw_bytes` and 2) by the time we
have stored all the image data, the start of it may be already gone from
the L1 cache.

After these changes, the image data is passed directly into
`ZlibStream`.

This commit is part of a bigger set of changes that avoids copying image
pixels across different buffers.  This commit alone gives notable
although relatively small performance improvements, but the real
motivation here is to make progress toward merging the bigger set of
changes and realizing their performance gains.  For more information
see the "Revisiting copy avoidance" part of the discussion at
image-rs#416 (comment)

Performance impact of this commit (2 measurement attempts):

* `decode/8x8-noncompressed.png` - runtime improved by 9.18% - 9.99%
* `128x128-noncompressed.png` - runtime improved by 7.89% - 8.51%
* `decode/Transparency.png` - runtime improved by 1.92% - 2.69%

No impact on the other test inputs:

* `decode/kodim02.png` - change within noise threshold (same result when
  rerun)
* `decode/kodim07.png` - no change detected (p = 0.06 or 0.17 > 0.05)
* `decode/kodim17.png` - runtime improved by 1.75%
  (change within noise threshold when rerun)
* `decode/kodim23.png` - runtime regressed by 2.15%
  (no change detected when rerun - p = 0.86 > 0.05)
* `Lohengrin_-_Illustrated_Sporting_and_Dramatic_News.png` - change
  within noise threshold (no change detected when rerun - p = 0.14 >
  0.05)

The move will help with follow-up work, where we want to use generated
PNGs in unit tests of `src/decoder/stream.rs`.

This has accidentally regressed in a recent commit and was caught during
PR review.