Tighten reference tolerances to measured difference plus one

Each threshold is the worst measured cross-adapter difference at its call
site, plus one at the measured precision: 0.0042 to 0.0043 for the backdrop
filters, 0.0044 to 0.0045 for the many-layer clip reduce, and 0.0097 to
0.0098 for the nested-region edge coverage.

Author: JimBobSquarePants

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

@@ -973,7 +973,7 @@ void DrawAction(DrawingCanvas canvas)
 
         // 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);
+        AssertBackendPairReferenceOutputs(provider, filter, defaultImage, nativeSurfaceImage, 0.0043F);
     }
 
     [WebGPUTheory]
@@ -2196,7 +2196,7 @@ void DrawAction(DrawingCanvas canvas)
 
         // 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);
+        AssertBackendPairReferenceOutputs(provider, null, defaultImage, nativeSurfaceImage, 0.0045F);
     }
 
     [WebGPUTheory]
@@ -4404,6 +4404,6 @@ static void DrawAction(DrawingCanvas canvas)
 
         // 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);
+        AssertBackendPairReferenceOutputs(provider, null, defaultImage, nativeSurfaceImage, 0.0098F);
     }
 }