#542: use bigger files for the tests to "hide" rounding issues:

The current implementation lacks accurrancy and prefers to be fast.
For usual color gradients that shouldn't matter: They're quite smooth, for the black-white gradient on very small length's it's easy to spot these rounding errors as they affect whole pixels one can count manually.

tests/ImageSharp.Tests/Drawing/FillLinearGradientBrushTests.cs

@@ -87,7 +87,7 @@ namespace SixLabors.ImageSharp.Tests.Drawing
         public void LinearGradientsWithDoubledStopsProduceDashedPatterns(
             float[] pattern)
         {
-            int width = 20;
+            int width = 200;
             int height = 10;
 
             // ensure the input data is valid