Initial commit: .NET MAUI Linux Platform

Complete Linux platform implementation for .NET MAUI with:

- 35+ Skia-rendered controls (Button, Label, Entry, CarouselView, etc.)
- Platform services (Clipboard, FilePicker, Notifications, DragDrop, etc.)
- Accessibility support (AT-SPI2, High Contrast)
- HiDPI and Input Method support
- 216 unit tests
- CI/CD workflows
- Project templates
- Documentation

🤖 Generated with Claude Code

Rendering/SkiaRenderingEngine.cs

@@ -0,0 +1,158 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using SkiaSharp;
+using Microsoft.Maui.Platform.Linux.Window;
+using System.Runtime.InteropServices;
+
+namespace Microsoft.Maui.Platform.Linux.Rendering;
+
+/// <summary>
+/// Manages Skia rendering to an X11 window.
+/// </summary>
+public class SkiaRenderingEngine : IDisposable
+{
+    private readonly X11Window _window;
+    private SKBitmap? _bitmap;
+    private SKCanvas? _canvas;
+    private SKImageInfo _imageInfo;
+    private bool _disposed;
+    private bool _fullRedrawNeeded = true;
+
+    public static SkiaRenderingEngine? Current { get; private set; }
+    public ResourceCache ResourceCache { get; }
+    public int Width => _imageInfo.Width;
+    public int Height => _imageInfo.Height;
+
+    public SkiaRenderingEngine(X11Window window)
+    {
+        _window = window;
+        ResourceCache = new ResourceCache();
+        Current = this;
+
+        CreateSurface(window.Width, window.Height);
+
+        _window.Resized += OnWindowResized;
+        _window.Exposed += OnWindowExposed;
+    }
+
+    private void CreateSurface(int width, int height)
+    {
+        _bitmap?.Dispose();
+        _canvas?.Dispose();
+
+        _imageInfo = new SKImageInfo(
+            Math.Max(1, width),
+            Math.Max(1, height),
+            SKColorType.Bgra8888,
+            SKAlphaType.Premul);
+
+        _bitmap = new SKBitmap(_imageInfo);
+        _canvas = new SKCanvas(_bitmap);
+        _fullRedrawNeeded = true;
+        
+    }
+
+    private void OnWindowResized(object? sender, (int Width, int Height) size)
+    {
+        CreateSurface(size.Width, size.Height);
+    }
+
+    private void OnWindowExposed(object? sender, EventArgs e)
+    {
+        _fullRedrawNeeded = true;
+    }
+
+    public void InvalidateAll()
+    {
+        _fullRedrawNeeded = true;
+    }
+
+    public void Render(SkiaView rootView)
+    {
+        if (_canvas == null || _bitmap == null)
+            return;
+
+        _canvas.Clear(SKColors.White);
+        
+        // Measure first, then arrange
+        var availableSize = new SKSize(Width, Height);
+        rootView.Measure(availableSize);
+        
+        rootView.Arrange(new SKRect(0, 0, Width, Height));
+        
+        // Draw the view tree
+        rootView.Draw(_canvas);
+        
+        // Draw popup overlays (dropdowns, calendars, etc.) on top
+        SkiaView.DrawPopupOverlays(_canvas);
+        
+        _canvas.Flush();
+
+        // Present to X11 window
+        PresentToWindow();
+    }
+
+    private void PresentToWindow()
+    {
+        if (_bitmap == null) return;
+
+        var pixels = _bitmap.GetPixels();
+        if (pixels == IntPtr.Zero) return;
+
+        _window.DrawPixels(pixels, _imageInfo.Width, _imageInfo.Height, _imageInfo.RowBytes);
+    }
+
+    public SKCanvas? GetCanvas() => _canvas;
+
+    protected virtual void Dispose(bool disposing)
+    {
+        if (!_disposed)
+        {
+            if (disposing)
+            {
+                _window.Resized -= OnWindowResized;
+                _window.Exposed -= OnWindowExposed;
+                _canvas?.Dispose();
+                _bitmap?.Dispose();
+                ResourceCache.Dispose();
+                if (Current == this) Current = null;
+            }
+            _disposed = true;
+        }
+    }
+
+    public void Dispose()
+    {
+        Dispose(true);
+        GC.SuppressFinalize(this);
+    }
+}
+
+public class ResourceCache : IDisposable
+{
+    private readonly Dictionary<string, SKTypeface> _typefaces = new();
+    private bool _disposed;
+
+    public SKTypeface GetTypeface(string fontFamily, SKFontStyle style)
+    {
+        var key = $"{fontFamily}_{style.Weight}_{style.Width}_{style.Slant}";
+        if (!_typefaces.TryGetValue(key, out var typeface))
+        {
+            typeface = SKTypeface.FromFamilyName(fontFamily, style) ?? SKTypeface.Default;
+            _typefaces[key] = typeface;
+        }
+        return typeface;
+    }
+
+    public void Clear()
+    {
+        foreach (var tf in _typefaces.Values) tf.Dispose();
+        _typefaces.Clear();
+    }
+
+    public void Dispose()
+    {
+        if (!_disposed) { Clear(); _disposed = true; }
+    }
+}