Handle FP errors in RoundLayoutValueUp.

If the `value` passed to `RoundLayoutValueUp` has a floating point precision error (e.g. 79.333333333333343 in the test case) then when it's multiplied by `dpiScale` and rounded up, it will be rounded up to the a value one greater than it should be.

Fix this by rounding the initial value to an arbitrary precision of 8 digits.

Fixes #8866

src/Avalonia.Base/Layout/LayoutHelper.cs

@@ -251,6 +251,17 @@ namespace Avalonia.Layout
         {
             double newValue;
 
+            // Round the value to avoid FP errors. This is needed because if `value` has a floating
+            // point precision error (e.g. 79.333333333333343) then when it's multiplied by
+            // `dpiScale` and rounded up, it will be rounded up to a value one greater than it
+            // should be.
+#if NET6_0_OR_GREATER
+            value = Math.Round(value, 8, MidpointRounding.ToZero);
+#else
+            // MidpointRounding.ToZero isn't available in netstandard2.0.
+            value = Math.Truncate(value * 1e8) / 1e8;
+#endif
+
             // If DPI == 1, don't use DPI-aware rounding.
             if (!MathUtilities.IsOne(dpiScale))
             {