This is akin to VST2's audioMasterGetBlockSize.
It returns the current nominal block size (think jack-buffersize).
It's not the only block size that may be used when calling run(), it's
just the normal one. The actual block sizes used may be larger or smaller
and may vary between successive calls of run().
This change became neccesary after 53e969e9. Some plugins expected
maxBlockLength to be the /current/ buffer-size and not all-time maxiumum.
Those plugins can now use nominalBlockLength.
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