Fix keep EXIF data -ke / --keep-exif with BytesIO buffer

[YellowTech]

As mentioned in #40 EXIF data is destroyed even when using the -ke flag. This is due to piexif.transplant expecting a subscriptable object (usually file path) which a BytesIO buffer itself is not. This is fixed by letting transplant use the buffer as an array and copying the resulting image into a new buffer which then replaces the old one.

This does not affect performance in any meaningful way, here are some Benchmarks on 50 giant (9mb) jpgs:

• New and correctly working:
  • Processed 50 files (320.2 MB) in 2.7s (18.3 f/s).
  • python -m optimize_images ./conv -ke 21.77s user 1.02s system 805% cpu 2.829 total

• Old with broken EXIF

  • Processed 50 files (320.2 MB) in 2.7s (18.2 f/s).
  • python -m optimize_images ./conv -ke 21.69s user 1.08s system 806% cpu 2.823 total

• Old without '-ke' parameter

  • Processed 50 files (320.2 MB) in 2.7s (18.6 f/s).
  • python -m optimize_images ./conv 21.66s user 0.90s system 821% cpu 2.747 total

[YellowTech]

Can we merge this? If this gets through I could work on HEIC support with pillow-heif.

[victordomingos]

Done. I will perform some testing before issuing the next release.m, so it may take a while before it comes to pip.