* chore: replace `ByteMemoryPool` usage with `MemoryOwner<byte>`
* refactor: `PixelConverter.ConvertR4G4ToR4G4B4A4()` - rename old `outputSpan` to `outputSpanUInt16`, reuse same output `Span<byte>` as newly-freed name `outputSpan`
* eliminate temporary buffer allocations
* chore, perf: use MemoryOwner<byte> instead of IMemoryOwner<byte>