Link to unicode math encoding spec.

Change-Id: I7d7967927ca463a67888a95d6c31156f254f521f
Allow rendering from unzipped bundle

Allow the renderer to consume an unzipped bundle file.  This saves CPU
cycles when the bundle was created on the same machine.  Due to the
use of hardlinks, the bundle directory in this case must be on the
same file system as the temp dir -- use the system environment
variable TMPDIR to specify an appropriate directory if necessary.

See change Iad77b766a12654876a7c140220df9a37dc25f6c6 for the
corresponding commit in the `mw-ocg-latexer` package.

Change-Id: Ieac2c6f4a53727f0162aa0fc10c5c6b2866bf0fa