Fix options calculation precedence

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -144,7 +144,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.height = image.Height;
 
             ImageMetadata metadata = image.Metadata;
-            PngMetadata pngMetadata = metadata.GetFormatMetadata(PngFormat.Instance);
+            PngMetadata pngMetadata = metadata.GetPngMetadata();
             PngEncoderOptionsHelpers.AdjustOptions<TPixel>(this.options, pngMetadata, out this.use16Bit, out this.bytesPerPixel);
             IQuantizedFrame<TPixel> quantized = PngEncoderOptionsHelpers.CreateQuantizedFrame(this.options, image);
             this.bitDepth = PngEncoderOptionsHelpers.CalculateBitDepth(this.options, image, quantized);

src/ImageSharp/Formats/Png/PngEncoderOptionsHelpers.cs

@@ -30,8 +30,10 @@ namespace SixLabors.ImageSharp.Formats.Png
             // Always take the encoder options over the metadata values.
             options.Gamma ??= pngMetadata.Gamma;
 
-            options.ColorType ??= SuggestColorType<TPixel>() ?? pngMetadata.ColorType;
+            // Use options, then check metadata, if nothing set there then we suggest
-            options.BitDepth ??= SuggestBitDepth<TPixel>() ?? pngMetadata.BitDepth;
+            // a sensible default based upon the pixel format.
+            options.ColorType ??= pngMetadata.ColorType ?? SuggestColorType<TPixel>();
+            options.BitDepth ??= pngMetadata.BitDepth ?? SuggestBitDepth<TPixel>();
 
             options.InterlaceMethod ??= pngMetadata.InterlaceMethod;
 
@@ -148,7 +150,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// Returns a suggested <see cref="PngColorType"/> for the given <typeparamref name="TPixel"/>
         /// This is not exhaustive but covers many common pixel formats.
         /// </summary>
-        private static PngColorType? SuggestColorType<TPixel>()
+        private static PngColorType SuggestColorType<TPixel>()
             where TPixel : struct, IPixel<TPixel>
         {
             return typeof(TPixel) switch
@@ -166,7 +168,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                 Type t when t == typeof(Rgb48) => PngColorType.Rgb,
                 Type t when t == typeof(Rgba64) => PngColorType.RgbWithAlpha,
                 Type t when t == typeof(RgbaVector) => PngColorType.RgbWithAlpha,
-                _ => default(PngColorType?)
+                _ => PngColorType.RgbWithAlpha
             };
         }
 
@@ -174,7 +176,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// Returns a suggested <see cref="PngBitDepth"/> for the given <typeparamref name="TPixel"/>
         /// This is not exhaustive but covers many common pixel formats.
         /// </summary>
-        private static PngBitDepth? SuggestBitDepth<TPixel>()
+        private static PngBitDepth SuggestBitDepth<TPixel>()
             where TPixel : struct, IPixel<TPixel>
         {
             return typeof(TPixel) switch
@@ -192,7 +194,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                 Type t when t == typeof(Rgb48) => PngBitDepth.Bit16,
                 Type t when t == typeof(Rgba64) => PngBitDepth.Bit16,
                 Type t when t == typeof(RgbaVector) => PngBitDepth.Bit16,
-                _ => default(PngBitDepth?)
+                _ => PngBitDepth.Bit8
             };
         }
     }

src/ImageSharp/Formats/Png/PngMetadata.cs

@@ -44,12 +44,12 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// Gets or sets the number of bits per sample or per palette index (not per pixel).
         /// Not all values are allowed for all <see cref="ColorType"/> values.
         /// </summary>
-        public PngBitDepth BitDepth { get; set; } = PngBitDepth.Bit8;
+        public PngBitDepth? BitDepth { get; set; }
 
         /// <summary>
         /// Gets or sets the color type.
         /// </summary>
-        public PngColorType ColorType { get; set; } = PngColorType.RgbWithAlpha;
+        public PngColorType? ColorType { get; set; }
 
         /// <summary>
         /// Gets or sets a value indicating whether this instance should write an Adam7 interlaced image.