Improve tkinter-based rotated image quality

[tmontes]

Facts

• Current implementation is simple, naive, and slow.
• Results are low-quality for two reasons:
  • No interpolation is in place.
  • Alpha control is all or nothing with current Tk version (will improve with TIP 166).
• In particular, from initial experiments, there is a huge quality different between the unrotated image and its rotated variations.

Thoughts

• Let's optionally support at least linear interpolation.
• It will be even slower, but image quality should be better.

PS: None of this is true when the optional Pillow 3rd party library is installed.