[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

The codec benchmarks created a new MemoryStream in each benchmark call. The timing then included the growth and copies of that stream, and Gen2 collections for its large arrays. In production, a caller would use a recycled stream. This PR applies the change from the BMP benchmarks in #3271 to the other codec benchmarks.

Each changed benchmark now uses one stream for both libraries:

• Encode benchmarks write into one MemoryStream that [GlobalSetup] creates. Each call sets Position = 0.
• Decode benchmarks read from one read-only MemoryStream over the input bytes. Each call sets Position = 0.
• [GlobalCleanup] disposes the stream.

The results now measure the codec, not the growth of a new stream.

Special cases:

• EncodeTiff calls SetLength(0) instead of Position = 0. With a rewound stream, the GDI+ TIFF encoder threw ExternalException ("A generic error occurred in GDI+.") for None and Ccitt1D.
• DecodeJpeg_ImageSpecific gives Skia an SKManagedStream(stream, false). SKCodec.Create(Stream) disposes the stream together with the codec.
• In DecodeTga, ImageMagick and ImageSharp read the shared stream. Pfim still reads the byte array.
• EncodeJxl used a new pooled ChunkedMemoryStream in each call. It now uses the shared MemoryStream. I updated Codecs/Jxl/README.md to match.

I ran each changed benchmark and replaced the result table in its comment. IdentifyJpeg, EncodeJxl, EncodeIndexedPng and EncodeGif_CoarsePaletteEncoder get their first table. The old table of DecodeJpegParseStreamOnly was outside the class. The new table is inside the class.

These benchmarks already reused their stream, so I did not change them: DecodeEncodeGif, DecodeJpeg, EncodeJpegComparison, EncodeJpegFeatures, EncodeJpegIcc and EncodeTiffIcc.

This change touches only benchmark code, so it adds no tests.

Changed files in tests/ImageSharp.Benchmarks/Codecs:

• Exr/DecodeExr.cs
• Gif/DecodeGif.cs
• Gif/EncodeGif.cs
• Heif/DecodeHeif.cs
• Heif/EncodeHeif.cs
• Jpeg/DecodeJpegParseStreamOnly.cs
• Jpeg/DecodeJpeg_ImageSpecific.cs
• Jpeg/IdentifyJpeg.cs
• Jxl/EncodeJxl.cs
• Jxl/README.md
• Png/DecodePng.cs
• Png/EncodeIndexedPng.cs
• Png/EncodePng.cs
• Tga/DecodeTga.cs
• Tga/EncodeTga.cs
• Tiff/DecodeTiff.cs
• Tiff/DecodeTiffIcc.cs
• Tiff/EncodeTiff.cs
• Webp/DecodeWebp.cs
• Webp/EncodeWebp.cs