A variable controlling whether the EGL window is allowed to be composited as transparent, rather than opaque.
Defined in SDL_hints.h
#define SDL_HINT_VIDEO_EGL_ALLOW_TRANSPARENCY "SDL_VIDEO_EGL_ALLOW_TRANSPARENCY"
Most window systems will always render windows opaque, even if the surface format has an alpha channel. This is not always true, however, so by default SDL will try to enforce opaque composition. To override this behavior, you can set this hint to "1".