`Sixel.GetCtrlSeqResponse()` has a bug still.

[teramako]

I thought I had fixed the problem by making Console.ReadKey() asynchronous in commit 5062423.
However, especially on WSL, it seems that the response is returned before waiting for input from Console.ReadKey(), and the standard input value is output to the terminal.

Perhaps we need to wait until the thread doing Console.ReadKey() is initialized and ready for before writing the terminal escape sequence to stdout.

Expected

Actual (sometimes)