Keep encoder output to non-seekable streams identical to a seekable save

[jafin]

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Description

When the destination stream cannot seek, ImageEncoder encodes into a ChunkedMemoryStream and copies the result to the destination. Two defects in ChunkedMemoryStream made that output differ from a seekable save, or made the save fail.

Overwriting data extended the stream. Write always added the bytes written to the length, also when it overwrote bytes after a seek back. The TIFF encoder seeks back to write each IFD offset (and BMP and ANI patch offsets the same way), so the buffered stream got longer by 4 bytes per patch. Those trailing bytes were uninitialized pooled memory, and they were copied to the destination. A TIFF save to a non-seekable stream, sync or async, therefore ended with stray bytes and did not match a seekable save.
Now the length only grows when a write ends after the current end of the data.

Seeking past the end failed or wrote at the wrong place. The ICO and CUR encoders seek forward past the directory, write the images, then go back and write the directory. On an empty buffer, SetPosition indexed chunk -1 and threw ArgumentOutOfRangeException, so saving ICO or CUR to a non-seekable stream always failed. With chunks allocated, a position at or after the end of the chunks was mapped to the last byte of the last chunk, so the next write started one byte early.
Now such a position maps to the start of the next chunk to allocate. Write finds the chunk again after it expands the buffer, if the position was past the end of the allocated chunks.

Tests

• ChunkedMemoryStreamTests: an overwrite after a seek back keeps the length and data. A write after a seek past the end (empty stream, exactly at a chunk boundary, and across chunks) lands at the right position.
• ImageTests.Save.ToNonSeekableStream_SyncAndAsync_MatchSeekableStream: for every built-in encoder, sync and async saves to a non-seekable stream are byte-identical to a seekable save. Before this change, this test failed for TIFF, ANI, ICO and CUR.
• TiffEncoderTests.TiffEncoder_ToNonSeekableStream_MatchesSeekableStream: a two-frame TIFF, so that more than one IFD offset is patched.

The Release build passes for net10.0. These test sets pass (3879 tests): Tiff, Ico, Cur, Ani and Bmp formats, ImageTests, GeneralFormatTests, ImageDecoder, and ChunkedMemoryStreamTests (which includes the non-seekable decode/encode integration tests over all test images).

Fixes #3244