Results of running microbenchmarks on author's machine:

```
$ bench --bench=unfilter --features=benchmarks,unstable -- --baseline=my_baseline filter=Paeth/bpp=3
...
unfilter/filter=Paeth/bpp=3
                        time:   [21.337 µs 21.379 µs 21.429 µs]
                        thrpt:  [546.86 MiB/s 548.14 MiB/s 549.22 MiB/s]
                 change:
                        time:   [-42.023% -41.825% -41.619%] (p = 0.00 < 0.05)
                        thrpt:  [+71.288% +71.895% +72.482%]
                        Performance has improved.
```

Results of running microbenchmarks on author's machine:

```
$ bench --bench=unfilter --features=unstable,benchmarks -- --baseline=my_baseline Paeth/bpp=6
...
unfilter/filter=Paeth/bpp=6
                        time:   [22.346 µs 22.356 µs 22.367 µs]
                        thrpt:  [1.0233 GiB/s 1.0238 GiB/s 1.0242 GiB/s]
                 change:
                        time:   [-24.033% -23.941% -23.852%] (p = 0.00 < 0.05)
                        thrpt:  [+31.323% +31.476% +31.637%]
                        Performance has improved.
```

This refactoring is desirable because:

* It removes a little bit of duplication between `unfilter_paeth3` and
  `unfilter_paeth6`
* It helps in a follow-up CL, where we need to use `paeth_step` from
  more places.

… or 6).

This CL loads RGB data using 4-bytes-wide loads (and RRGGBB data using
8-byte-wide loads), because:

* This is faster as measured by the microbenchmarks below
* It doesn't change the behavior - before and after these changes we
  were ignoring the 4th SIMD lane when processing RGB data (after this
  change the 4th SIMD lane will contain data from the next pixel, before
  this change it contained a 0 value)
* This is safe as long as we have more than 4 bytes of remaining input
  data (we have to fall back to a 3-bytes-wide load for the last pixel).

Results of running microbenchmarks on the author's machine:

```
$ bench --bench=unfilter --features=unstable,benchmarks -- --baseline=simd1 Paeth/bpp=[36]
...
unfilter/filter=Paeth/bpp=3
                        time:   [18.755 µs 18.761 µs 18.767 µs]
                        thrpt:  [624.44 MiB/s 624.65 MiB/s 624.83 MiB/s]
                 change:
                        time:   [-16.148% -15.964% -15.751%] (p = 0.00 < 0.05)
                        thrpt:  [+18.696% +18.997% +19.258%]
                        Performance has improved.
...
unfilter/filter=Paeth/bpp=6
                        time:   [18.991 µs 19.000 µs 19.009 µs]
                        thrpt:  [1.2041 GiB/s 1.2047 GiB/s 1.2052 GiB/s]
                 change:
                        time:   [-15.161% -15.074% -14.987%] (p = 0.00 < 0.05)
                        thrpt:  [+17.629% +17.750% +17.871%]
                        Performance has improved.
```