Fix Mac crash when creating fullscreen window introduced in 9d43403e9fc5.

makeKeyAndOrderFront: was sending three KVO transitions for isVisible,
for false -> true, true -> false, and then false -> true. This was
causing an infinite recursion.

We now suspend monitoring of the KVO before makeKeyAndOrderFront, then
resume afterwards and send any changes in isVisible's state.
Make Cocoa_ShowMessageBox work in background threads.
Send mouse leave updates for Windows and X11.

We now generate a final SDL_MOUSEMOTION before the cursor leaves the
window, followed by a SDL_WINDOWEVENT_LEAVE.
Forgot to add this file to Mac crash fix.
SDL_GL_DeleteContext would leave an invalid current_glctx.

Calling SDL_GL_DeleteContext wouldn't update current_glctx, so you could
end up with use-after-free and other goodies when you deleted a context.
Add multimon & mouse tracking info to SDL_test_common.