Merge separate assert file

Author: JimBobSquarePants

src/ImageSharp/Formats/Icon/IconAssert.cs

@@ -175,14 +175,14 @@ protected void ReadHeader(Stream stream)
         Span<byte> buffer = stackalloc byte[IconDirEntry.Size];
 
         // ICONDIR
-        _ = IconAssert.EndOfStream(stream.Read(buffer[..IconDir.Size]), IconDir.Size);
+        _ = CheckEndOfStream(stream.Read(buffer[..IconDir.Size]), IconDir.Size);
         this.fileHeader = IconDir.Parse(buffer);
 
         // ICONDIRENTRY
         this.entries = new IconDirEntry[this.fileHeader.Count];
         for (int i = 0; i < this.entries.Length; i++)
         {
-            _ = IconAssert.EndOfStream(stream.Read(buffer[..IconDirEntry.Size]), IconDirEntry.Size);
+            _ = CheckEndOfStream(stream.Read(buffer[..IconDirEntry.Size]), IconDirEntry.Size);
             this.entries[i] = IconDirEntry.Parse(buffer);
         }
 
@@ -232,4 +232,14 @@ private IImageDecoderInternals GetDecoder(bool isPng)
             UseDoubleHeight = true,
         });
     }
+
+    private static int CheckEndOfStream(int v, int length)
+    {
+        if (v != length)
+        {
+            throw new InvalidImageContentException("Not enough bytes to read icon header.");
+        }
+
+        return v;
+    }
 }

src/ImageSharp/Formats/Icon/IconDecoderCore.cs

@@ -90,7 +90,8 @@ public void Encode<TPixel>(
                     // Only 32bit Png supported.
                     // https://devblogs.microsoft.com/oldnewthing/20101022-00/?p=12473
                     BitDepth = PngBitDepth.Bit8,
-                    ColorType = PngColorType.RgbWithAlpha
+                    ColorType = PngColorType.RgbWithAlpha,
+                    CompressionLevel = PngCompressionLevel.BestCompression
                 },
                 _ => throw new NotSupportedException(),
             };