Add an extension to alter the block alignment for buffer unpack/pack ops

This is for unpacking (reading, e.g. alBufferData) and packing (writing, e.g.
alGetBufferSamplesSOFT) operations. The alignments are specified in sample
frames, with 0 meaning the default (65 for IMA4, 1 otherwise). IMA4 alignment
must be a multiple of 8, plus 1 (e.g. alignment = n*8 + 1), otherwise an error
will occur during (un)packing. Chenging the block alignment does not affect
already-loaded sample data, only future unpack/pack operations... so for
example, this is perfectly valid:

// Load mono IMA4 data with a block alignment of 1024 bytes, or 2041 sample
// frames.
alBufferi(buffer, AL_UNPACK_BLOCK_ALIGNMENT_SOFT, 2041);
alBufferData(buffer, AL_FORMAT_MONO_IMA4, data, data_len, srate);
alBufferi(buffer, AL_UNPACK_BLOCK_ALIGNMENT_SOFT, 0);

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