quasi88: switch upstream repository, upgrade to 0.7.1

[cmitu]

Switched the Quasi88 upstream repository to the "Quasi88Kai" fork [1], since it has some new developments and also supports SDL2.

Changes in the fork:

* General updates to the build system and migration to CMake
* All files converted to UTF-8
* Many improvements in Windows, X11, GTK and SDL backends
* Add a RetroAchievements-compatible version "RAQUASI88" (only for Windows at this time)

Had to apply a build patch since:

• SDL2 2.0.10 libraries do not provide the SDL:SDL aliases in the sdl2-config.cmake
• Buster's GCC (8) is implementing the filesystem C++17 API in libstdc++fs

[1] https://github.com/winterheart/quasi88

[joolswills]

Thank you.