* Disable OpenGL for SDL2 window backend and introduce ANGLE egl_backend
* Allow to build with legacy OpenGL provided from Apple and add docs
* Apply code review suggestions
* Bump SDL2 version in build dependencies script
* Use pre-release 2.7.1 for sdl_image
* Link for pre-releases is different
* sdl_image 2.8.0 has now been released
* Update kivy_deps.sdl2 and kivy_deps.sdl2-dev to 0.7.0 (to reflect the same changes made in Linux and macOS build scripts)