Add Letterbox plugin

[randall-hash]

Adds Letterbox — renders the game at a smaller, configurable size centered in black space for the Resizable - Classic layout (a compact play area floating in black on large/ultrawide monitors).

Source: https://github.com/randall-hash/letterbox

How it works: it resizes the game engine component inside ClientPanel (not the canvas) and lets the panel's existing black background show in the margins. Because the engine genuinely renders at the smaller size, the interface reflows and mouse hit-detection stays correct.

Compliance notes:

• No network/HTTP, no IP submission to any third party (so no warning= line), no file I/O.
• No input injection, no menu entries / server actions, no reflection, Java 11.
• Listeners/timer/layout are all torn down in shutDown (the game returns to filling the window).

One thing I want to flag proactively re: the "no moving or resizing click zones for 3D components" guideline: Letterbox resizes the entire viewport uniformly — click targets get smaller, never larger, so it confers no targeting advantage. The built-in Stretched Mode is the precedent for whole-viewport scaling. Happy to adjust if you'd prefer a different approach.

[runelite-github-app]

New plugin letterbox: https://github.com/randall-hash/letterbox/tree/13f610b858529fc3ca9e0f7d815d1ac1617845e9

[runelite-github-app]

Internal use only: Reviewer details Maintainer details

[randall-hash]

Author here. I noticed the “waiting for author” label but don’t see a comment on the PR, so I wanted to check what you’d like me to address.