Try out some SDL1 tests for the overlay module.

[MyreMylar]

The overlay module is pretty neglected, probably because it doesn't exist in SDL2 but we have one issue open about it here:

#1199

And I ran across a potential porting guide here:

https://stackoverflow.com/questions/17579286/sdl2-0-alternative-for-sdl-overlay

So, I thought I'd see if I could get some tests going for SDL1 and see if it will at least work under SDL1.

• first issue: it occasionally crashes with my test data running on Python 3.8 and Pygame 1.9.6. I get this DeprecationWarning:
  DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats overlay.display(raw). I followed the guidance in this issue: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats python-pillow/Pillow#3750 and made a few modifications to overlay.c to see if that will fix it.

[MyreMylar]

This looks useful for understsanding the weird pixel formats used by overlay:

http://rawpixels.net/

[MyreMylar]

The end result of this testing is that we can get the SDL1 build up and running on CI again without too much work, so I'm going to do that first in a separate PR - then revisit this one (and probably start it over again).