Add experimental Kotlin/Native Linux backend (X11)

[kdroidFilter]

Description

This PR introduces an experimental Kotlin/Native Linux backend based on X11, allowing Skiko to run natively on Linux without relying on AWT or the JVM.

The main goal is to establish a minimal, maintainable native Linux foundation aligned with Skiko’s existing architecture, serving as a solid base for future Linux backends such as Wayland or EGL.

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What’s Included

X11 Window Host

• X11SkikoWindow

  • Native event loop
  • Resize handling
  • Basic pointer and keyboard input
  • Cursor support
  • Window manager close handling

Linux SkiaLayer Implementation

• Attach and detach lifecycle for native windows
• needRender and redraw integration
• Picture recording in update() and replay in draw()

Native Rendering Backends with Automatic Fallback

• OpenGL via GLX as the primary GPU rendering path
• Software rendering via XPutImage as a fallback

Build and Toolchain Wiring

• X11 and GLX cinterop for linuxX64 and linuxArm64
• Linux native enablement set to auto on Linux hosts
• Linux linker adjustments (-ldl)
• __libc_single_threaded stub to unblock Kotlin/Native toolchain compatibility

Linux Platform Utilities

• Best-effort open-URI implementation
• Clipboard support using Wayland and X11 tools
• System theme detection for GNOME, KDE, XFCE, and others
• Minimal test to ensure theme detection does not crash

Sample Updates

• Native Linux executable configuration
• runNative Gradle task
• Documentation updates
• SKIKO_RENDER_API selection for testing

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How to Test (Linux)

Dependencies (Debian / Ubuntu)

sudo apt-get install libx11-dev libxrandr-dev libgl1-mesa-dev

Run

cd samples/SkiaMultiplatformSample
./gradlew runNative

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Notes / Scope

• This backend currently targets X11 only.
• OpenGL via GLX is the primary and intended GPU rendering path.
• Software rendering is provided as a fallback for unsupported environments.

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Roadmap (Out of Scope for This PR)

• Wayland backend based on EGL for modern Linux environments and true zero-copy rendering
• Additional native GPU backends as follow-up work