[OpenJDK 2D-Dev] RFR: 8186317: Cache font layout tables for use by harfbuzz

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8186317
Webrev: http://cr.openjdk.java.net/~prr/8186317/

In the ICU code path, we cache the layout tables in native memory so that
when requested by ICU we can just hand the pointer, saving the overhead
of copying the Java array for every call to layout.

We weren't doing this for harfbuzz and this webrev fixes that.

I noticed harfbuzz always retrieves the 'head' table so I decided to add 
that to the list of
cached tables.

The overall result is something like 25-30% performance gain.

-phil.