Fix VP8X reserved bytes use Position += 3 instead of Write, causing 3-byte truncation in alpha WebP images

[jrlost]

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Description

This addresses the issue outlined here: #3035

Changed this stream advancement

        stream.Position += 3; // Reserved bytes

to instead advance the stream while also setting it

Span<byte> reserved = stackalloc byte[3];
stream.Write(reserved);

I have verified this resolves the issue addressed in 3035.

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