Admit cross-adapter rounding in reference-output tolerances

The reference images are rendered on one adapter; conforming adapters differ
by one LSB across a small fraction of pixels, and by antialiased edge coverage
on region boundaries. The measured differences are 0.003-0.010 percent under
the comparer's metric, against thresholds of 0.0005-0.0006; the new values
carry roughly double the measured headroom while staying far below the 0.39
percent a single-LSB difference on every pixel would produce.

Author: JimBobSquarePants

tests/ImageSharp.Drawing.Tests/Processing/Backends/WebGPUDrawingBackendTests.cs

@@ -970,7 +970,10 @@ void DrawAction(DrawingCanvas canvas)
 
         DebugSaveBackendPair(provider, filter, defaultImage, nativeSurfaceImage);
         AssertBackendPairSimilarity(defaultImage, nativeSurfaceImage, 0.03F);
-        AssertBackendPairReferenceOutputs(provider, filter, defaultImage, nativeSurfaceImage, 0.0006F);
+
+        // Reference outputs are rendered on one adapter; other conforming adapters round
+        // within one LSB across a small fraction of pixels.
+        AssertBackendPairReferenceOutputs(provider, filter, defaultImage, nativeSurfaceImage, 0.01F);
     }
 
     [WebGPUTheory]
@@ -2190,7 +2193,10 @@ void DrawAction(DrawingCanvas canvas)
 
         DebugSaveBackendPair(provider, null, defaultImage, nativeSurfaceImage);
         AssertBackendPairSimilarity(defaultImage, nativeSurfaceImage, 0.005F);
-        AssertBackendPairReferenceOutputs(provider, null, defaultImage, nativeSurfaceImage, 0.0006F);
+
+        // Reference outputs are rendered on one adapter; other conforming adapters round
+        // within one LSB across a small fraction of pixels.
+        AssertBackendPairReferenceOutputs(provider, null, defaultImage, nativeSurfaceImage, 0.01F);
     }
 
     [WebGPUTheory]
@@ -4395,6 +4401,9 @@ static void DrawAction(DrawingCanvas canvas)
 
         DebugSaveBackendPair(provider, null, defaultImage, nativeSurfaceImage);
         AssertBackendPairSimilarity(defaultImage, nativeSurfaceImage, 0.108F);
-        AssertBackendPairReferenceOutputs(provider, null, defaultImage, nativeSurfaceImage, 0.0005F);
+
+        // Reference outputs are rendered on one adapter; other conforming adapters differ by
+        // rounding and antialiased edge coverage on a small fraction of pixels.
+        AssertBackendPairReferenceOutputs(provider, null, defaultImage, nativeSurfaceImage, 0.02F);
     }
 }