sdl12-compat testsprite issues

[ritalat]

Testsprite from sdl12-compat fails to render correctly when chained through sdl2-compat. The program also segfaults on exit and if SDL3 is not found.

This issue does not happen with real SDL2.

[icculus]

Not sure what's going on in that screenshot, but I needed the latest in sdl2-compat to fix an assertion failure.

testsprite works after that in Debug builds for me (but crashes with a double-free on exit). In release builds I just get a black window and a segfault on quit. So there's still work to do here.

Other software-rendered tests from sdl12-compat work, like testoverlay and testbitmap, so it might be something testsprite itself is doing.