Per our previous discussion, this patch adds logic to minimize the window on Alt+Tab if:
- The window has keyboard grab
- The window is full-screen
- SDL_HINT_NO_GRABBED_ALT_TAB_HANDLER is 0 or unset

I'm not sure if there's a different place you'd rather me put this code, but I stuck it in SDL_SendKeyboardKeyInternal().

I went with minimizing the window instead of something like manually releasing the keyboard grab because I figure it will be less disturbing to the app. If we manually released the keyboard grab, the application would have to figure out that the grab disappeared from under it and regrab when it gains focus again. It would also require the user to press Alt+Tab twice to get the desired behavior, since the first would be eaten by the keyboard grab. By using minimize, we can let the focus leaving naturally release the grab and regrab on subsequent focus gain.