feat: DX12 DirectComposition path for per-pixel alpha transparency

[kolkov]

feat: DX12 DirectComposition path for per-pixel alpha transparency

Problem

DX12 HAL uses CreateSwapChainForHwnd (surface.go:85) which supports only CompositeAlphaModeOpaque. Per-pixel alpha transparency (rounded windows, tray popups, overlays) requires CompositeAlphaModePremultiplied, which is only available via CreateSwapChainForComposition (DirectComposition).

Confirmed from Rust wgpu (adapter.rs:1071):

SurfaceTarget::WndHandle(_) => vec![wgt::CompositeAlphaMode::Opaque],

Rust wgpu Reference

Rust wgpu already implements DirectComposition in dcomp.rs (~120 LOC):

1. DCompositionCreateDevice2() from dcomp.dll — create DirectComposition device
2. CreateTargetForHwnd(hwnd) — bind to window
3. CreateVisual() — create visual
4. SetRoot(visual) — set as root
5. CreateSwapChainForComposition(visual) instead of CreateSwapChainForHwnd(hwnd)

Proposed Implementation

Add alternative surface creation path in hal/dx12/surface.go:

• If SurfaceConfiguration.AlphaMode == Premultiplied → use DirectComposition path
• Otherwise → use current HWND path (no change for existing users)

Load dcomp.dll lazily via goffi (same pattern as other Win32 DLLs). ~120-150 LOC.

Impact

• Enables per-pixel alpha on DX12 (rounded tray popups, overlays, HUDs)
• No breaking changes — HWND path remains default
• Triggered by gogpu#361 (@shaolei — DeskCalendar), ADR-060

[kolkov]

wgpu side: done (v0.30.35, PR #299).

Remaining work is in gogpu — @shaolei has two PRs open for the windowing layer:

• gogpu#419 — WithTransparent config + premultiplied swapchain alpha
• gogpu#420 — platform window styles (DwmBlurBehind, X11 ARGB, macOS)

wgpu v0.30.35 is already bumped in gogpu (PR #417, merged). Once gogpu PRs land, end-to-end per-pixel alpha will be testable.

Keeping this open until visual verification is complete.