RFR: 8129582: Controls slow considerably when displaying RTL-languages text on Linux

[Phil Race]

Please review this fix for a performance problem in Pango Text Layout.
The root cause was pointed out by itaisha at gmail.com, that the fontmap
is an expensive thing to create, and we were recreating it on each call 
to layout.
By caching it we can avoid this huge overhead.

Bug : https://bugs.openjdk.java.net/browse/JDK-8129582
webrev : http://cr.openjdk.java.net/~prr/8129582/

-phil.