Overlay flicker line with Kitty image renderer

[tmustier]

Summary

Kitty overlay rendering causes a 1-line flicker where the overlay background differs from the underlying Pi background. Attempted mitigation to skip unchanged frames made flicker worse/spread.

Findings

• Flicker appears on the overlay boundary when background colors differ.
• Worse after fix(renderer): skip unchanged kitty frames (commit cb945a6).
• Reverted in follow-up commit.

Suspected cause

• Overlay compositing clears overlay lines each render; Kitty image updates + cursor repositioning cause a visible line of background mismatch.
• Likely requires changes in pi-tui overlay compositing for image lines or a different render path (non-overlay or full-screen draw).

Repro

1. Run NES in image mode with a Pi background color visible behind the overlay.
2. Observe a flicker line at the overlay boundary.

Notes

• The previous cursor-move attempt (moveDown after ESC_G) caused full-screen flicker; reverted.