AG_WindowShow() fails on macOS Sonoma

[kwhr0]

On macOS Sonoma, SDL_GL_CreateContext() fails after calling SDL_GetWindowSurface().
https://discourse.libsdl.org/t/sdl-window-opengl-on-mac-sonoma-sdl-2-28-5-not-working/48866

The error message is as follows.

SDL_GL_CreateContext: The specified window isn't an OpenGL window

This causes AG_WindowShow() to fail.

I patched the following as a first aid, but it is not universal.
https://github.com/kwhr0/libagar/tree/first_aid