Add a semi-automatic test for #221

[smcv]

This needs to be given a valid font as a command-line argument, but is otherwise automatic.

---

Includes #223.

I haven't wired this up to the CMake build system or the CI, because I don't want to put this on the critical path for 2.20.0, but it does seem to work.

[smcv]

Given how long it took to get the SDL_image test passing reliably, if @slouken is doing 2.20.0 soon then I'm inclined to wait for 2.21.x to add this - it's just not as high a priority as the CMake build system or getting a release out.

[smcv]

Given how long it took to get the SDL_image test passing reliably, if @slouken is doing 2.20.0 soon then I'm inclined to wait for 2.21.x to add this - it's just not as high a priority as the CMake build system or getting a release out.

[slouken]

I'm okay with adding this now. The SDL_image test found bugs that were good to fix for the next release, so if that happens here, that makes me happy. :)

[smcv]

OK, I'll try to get it integrated into CI in a way that isn't randomly regressing, but please don't block on this for 2.20.

(Also please don't imagine that this is a thorough test - it doesn't even render any text.)