Add CPU pixel presentation

[treeform]

Summary

• Add a -d:useCpu Windy backend path that creates windows without an OpenGL context.
• Add window.presentPixels(image) for CPU-rendered Pixie images.
• Present CPU pixels through Win32 StretchDIBits on Windows.
• Present CPU pixels through a plain AppKit NSView on macOS.
• Keep CPU smoke coverage for both Windows and macOS.

Validation

• nim r -d:useCpu tests/test_cpu_pixels.nim on macOS
• nim check --os:windows -d:useCpu tests/test_cpu_pixels.nim
• git diff --check