Fix touch coordinate scaling in sample app: multiply by DPI so lines draw where the user touches

Touch events from JS arrive in CSS pixels, but the SkiaSharp canvas surface
renders at device pixel ratio when IgnorePixelScaling is false (default).
The app now scales touch coordinates, stroke widths, and legend elements by
DPI. This is done in the app per the reviewer's guidance — the library
should not apply scaling.

Co-authored-by: mattleibow <1096616+mattleibow@users.noreply.github.com>

Author: Copilot

samples/SkiaSharpDemo.Blazor/Pages/Touch.razor

@@ -129,26 +129,32 @@
         if (_log.Count > MaxLogEntries)
             _log.RemoveAt(_log.Count - 1);
 
+        // Scale CSS-pixel coordinates to canvas pixels (device DPI).
+        // Touch events arrive in CSS pixels, but the SkiaSharp surface
+        // renders at device pixel ratio when IgnorePixelScaling is false.
+        var dpi = _canvasView?.Dpi ?? 1f;
+        var scaledLocation = new SKPoint(e.Location.X * dpi, e.Location.Y * dpi);
+
         // Drawing strokes
         switch (e.ActionType)
         {
             case SKTouchAction.Pressed:
                 var stroke = new Stroke { DeviceType = e.DeviceType };
-                stroke.Points.Add(e.Location);
+                stroke.Points.Add(scaledLocation);
                 _strokes.Add(stroke);
                 _activeStrokes[e.Id] = stroke;
                 break;
 
             case SKTouchAction.Moved:
                 if (_activeStrokes.TryGetValue(e.Id, out var active))
-                    active.Points.Add(e.Location);
+                    active.Points.Add(scaledLocation);
                 break;
 
             case SKTouchAction.Released:
             case SKTouchAction.Cancelled:
                 if (_activeStrokes.TryGetValue(e.Id, out var finished))
                 {
-                    finished.Points.Add(e.Location);
+                    finished.Points.Add(scaledLocation);
                     _activeStrokes.Remove(e.Id);
                 }
                 break;
@@ -165,10 +171,13 @@
         var canvas = e.Surface.Canvas;
         canvas.Clear(SKColors.White);
 
+        // Scale stroke width and legend by DPI so they look correct at device resolution
+        var dpi = _canvasView?.Dpi ?? 1f;
+
         using var paint = new SKPaint
         {
             IsAntialias = true,
-            StrokeWidth = 3,
+            StrokeWidth = 3 * dpi,
             Style = SKPaintStyle.Stroke,
             StrokeCap = SKStrokeCap.Round,
             StrokeJoin = SKStrokeJoin.Round,
@@ -188,17 +197,17 @@
             canvas.DrawPath(path, paint);
         }
 
-        // Legend in top-left corner
-        using var legendFont = new SKFont { Size = 12, Edging = SKFontEdging.SubpixelAntialias };
+        // Legend in top-left corner (scaled by DPI)
+        using var legendFont = new SKFont { Size = 12 * dpi, Edging = SKFontEdging.SubpixelAntialias };
         using var legendPaint = new SKPaint { IsAntialias = true };
-        var y = 16f;
+        var y = 16f * dpi;
         foreach (var (name, _, color) in DeviceColors)
         {
             legendPaint.Color = color;
-            canvas.DrawCircle(12, y - 4, 5, legendPaint);
+            canvas.DrawCircle(12 * dpi, y - 4 * dpi, 5 * dpi, legendPaint);
             legendPaint.Color = SKColors.Black;
-            canvas.DrawText(name, 22, y, SKTextAlign.Left, legendFont, legendPaint);
-            y += 18;
+            canvas.DrawText(name, 22 * dpi, y, SKTextAlign.Left, legendFont, legendPaint);
+            y += 18 * dpi;
         }
     }