Add optional Metal GPU-accelerated terminal renderer

[trsdn]

Summary

Adds an optional Metal-based rendering backend as an alternative to CoreGraphics. The renderer is opt-in — existing behavior is completely unchanged unless the user explicitly sets tv.renderer = MetalTerminalRenderer().

Changes

New files (all in Sources/SwiftTerm/Rendering/):

File	Lines	Purpose
TerminalRenderer.swift	59	Protocol abstracting rendering backends
CoreGraphicsRenderer.swift	44	Thin wrapper around existing drawTerminalContents()
MetalTerminalRenderer.swift	~1050	GPU-accelerated renderer with instanced drawing
GlyphAtlas.swift	~350	CoreText → MTLTexture shelf-packed glyph cache
ShaderTypes.h	35	Shared Metal/Swift struct definitions
TerminalShaders.metal	~150	Reference shaders (compiled at runtime for SPM)

Modified files:

• MacTerminalView.swift (+25 lines): renderer property, default CoreGraphicsRenderer in setup(), draw() dispatch

Architecture

TerminalRenderer (protocol)
├── CoreGraphicsRenderer   — wraps existing drawTerminalContents()
└── MetalTerminalRenderer  — GPU-accelerated path
    ├── GlyphAtlas         — CoreText → MTLTexture cache (2048×2048 .r8Unorm)
    └── Metal Shaders      — 4 instanced draw passes

Rendering pipeline: Background → Text → Decorations → Images (all instanced, single draw call per pass).

Key Design Decisions

1. Retina-native glyph atlas — glyphs rasterized at backingScaleFactor (2x on Retina) for 1:1 texel-to-pixel mapping. No bilinear blur.
2. Step threshold — step(0.35, alpha) in fragment shader for crisp binary edges. CoreText anti-aliasing captures full glyph shape, threshold converts to sharp output.
3. Pixel-snapped grid — floor() on all cell origins prevents sub-pixel seams.
4. No font smoothing in atlas — setShouldSmoothFonts(false) + setShouldAntialias(true). Smoothing in a grayscale context creates wide halos that look washed out.
5. Self-contained blink timer — MetalTerminalRenderer manages its own blink state, no changes needed to existing text attribute logic.
6. Graceful fallback — MetalTerminalRenderer.isAvailable returns false on VMs/CI without GPU.
7. SPM compatible — shaders compiled from source string at runtime since SPM cannot compile .metal files.

Usage

let terminal = LocalProcessTerminalView(frame: frame)
if MetalTerminalRenderer.isAvailable {
    terminal.renderer = MetalTerminalRenderer()
}

What's NOT changed

• Default rendering path (CoreGraphics) is untouched
• All 373 existing tests pass
• No API changes to existing public types
• No new dependencies

Tested with

• ✅ Text rendering (bold, italic, underline, strikethrough, blink, dim, inverse)
• ✅ Full 256-color + TrueColor
• ✅ Wide characters (CJK)
• ✅ Box drawing and block elements
• ✅ Selection overlay + all cursor styles
• ✅ Dynamic resize
• ✅ Retina displays
• ✅ Sixel/Kitty image rendering
• ✅ Scrollback buffer

Closes #479

[migueldeicaza]

I have attempted this a few times, the most recent one is in my gpu branch, there are a lot of small and subtle issues lurking behind it. Check it and let me know if there are improvements we should bring from yours to mine, but I am a bit hesitant to bring yours given the wide spectrum of small things I had to address along the way.

[migueldeicaza]

My last update was about 3 weeks ago, I will try to merge main into that branch, so we can compare apples to apples.

[migueldeicaza]

Ok, I merged my changes into the gpu branch, would love if you could review if my branch is missing any features from the work you did.