Add support for synchronized output to prevent tearing

If I run an application which continuously sends large chunks of text + ANSI control sequences to stdout, I sometimes see the cursor jump to the middle of the rendering. The application is enabling mode 2026 and sending the end-of-frame sequences. VHS should track whether this mode is enabled and not re-render a frame until it sees the end-of-frame sequence.

More details available at https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036