Kitty graphics: Use local file mode when available

[cryptocode]

Currently base64 encoded direct transfer is used for the pixel data. This has the upside of making Doom playable over SSH.

However, testing shows that file mode (local only) can be significantly faster, since the file containing the current frame will always be in the OS file cache and there's no base64 encoding/decoding going on.

It's preferable that file mode is detected (Kitty graphics responds with an error if file is not found), but if not possible a command line flag is acceptable.