Address terminal rendering issues with webgl, improve testing gpu acceleration

[anthonykim1]

terminal.integrated.gpuAcceleration
Been seeing multiple instances of rendering glitch/break apart especially when using TUI CLI such as Copilot CLI in terminal editor, etc.

Wanting to have stress test, better coverage for terminal gpu acceleration, maybe this should also add test in xterm.js directly as well.

[anthonykim1]

Opened xtermjs/xterm.js#6038 for the xterm-side WebGL atlas investigation.

Current finding: at least two xterm.js WebGL atlas failure modes:

• shared atlas page changes not being observed by every renderer
• atlas page growth exceeding the renderer texture cap

planning to split the current draft xterm.js PR into smaller test-first fixes rather than treating the large draft PR as the final review shape.