0.7 -- 2022/12/22 -- (breaking change) remove Graphics dependency

• Graphics is now optional. Use the oplot-graphics package to restore it.
  We use a functor signature for this.

• Slight breaking change: module Internal is now part of Plt
  (sounder signature). Just use Oplot.Plt.Internal instead of
  Oplot.Internal and everything should be fine.

Better Mac OS support:

• fix closing window problem from the toplevel
  (https://github.com/sanette/oplot/issues/3, thanks @anentropic)

• support for crisp graphics on retina screen

0.6 -- 2022/11/22 -- upgrade to SDL2

Drop the dependency on the old SDL12 library (and hence ocamlsdl),
switch to tsdl, tsdl-image, tsdl-ttf.

0.5 -- last version with SDL12

0.1, 0.2, 0.3, 0.4 -- 2007 (yes, that's old)