Increase LimitAllocatorBufferCapacity to 400 for RLE tests.

Add info to ImageFormatException that this may happen for large RLE images

src/ImageSharp/Formats/Bmp/BmpDecoder.cs

@@ -45,7 +45,7 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
                 // TODO: use InvalidImageContentException here, if we decide to define it
                 // https://github.com/SixLabors/ImageSharp/issues/1110
-                throw new ImageFormatException($"Can not decode image. Failed to allocate buffers for possibly degenerate dimensions: {dims.Width}x{dims.Height}.", ex);
+                throw new ImageFormatException($"Can not decode image. Failed to allocate buffers for possibly degenerate dimensions: {dims.Width}x{dims.Height}. This error can happen for very large RLE bitmaps, which are not supported.", ex);
             }
         }
 

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -261,7 +261,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         {
             if (enforceDiscontiguousBuffers)
             {
-                provider.LimitAllocatorBufferCapacity().InBytes(100);
+                provider.LimitAllocatorBufferCapacity().InBytes(400);
             }
 
             using (Image<TPixel> image = provider.GetImage(new BmpDecoder { RleSkippedPixelHandling = RleSkippedPixelHandling.FirstColorOfPalette }))
@@ -283,7 +283,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Bmp
         {
             if (enforceNonContiguous)
             {
-                provider.LimitAllocatorBufferCapacity().InBytes(50);
+                provider.LimitAllocatorBufferCapacity().InBytes(400);
             }
 
             using (Image<TPixel> image = provider.GetImage(new BmpDecoder { RleSkippedPixelHandling = RleSkippedPixelHandling.Black }))