Fix partial flush on flush_or_close()

[mtth-bfft]

BROTLI_OPERATION_FLUSH must be re-tried until no more data is available as input and internal buffers are empty (this can take more than one call). Otherwise, the writer is left in a "flushing" state which will make further writes fail.

The documentation on how to use BROTLI_OPERATION_FLUSH (https://brotli.org/encode.html#aed2 , https://brotli.org/encode.html#a512) mentions the compressor should be called until it has no available input data and BrotliEncoderHasMoreOutput() returns false:

	Under some circumstances (e.g. lack of output stream capacity) this operation would require several calls to BrotliEncoderCompressStream
	[...]
	Warning
	When flushing and finishing, op should not change until operation is complete; input stream should not be swapped, reduced or extended as well.

The current implementation calls BrotliEncoderCompressStream() once and always returns Ok(), which may leave the compressor in BROTLI_OPERATION_FLUSH state and make future write operations return an Err() (and allow the caller to modify the input stream, leaving room for unexpected compression results).

This patch should fix #73

[CLAassistant]

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