[OpenJDK 2D-Dev] RFR: 8238942: Rendering artifacts with LCD text and fractional metrics

[Philip Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8238942
webrev: http://cr.openjdk.java.net/~prr/8238942/

As discussed in the bug, we are missing padding in the code that takes
an LCD glyph from freetype and caches it in the 2D glyph cache.

This padding is used in subpixel positioning with fractional metrics on.

I've run automated regression tests as well as doing extensive manual 
testing
and so far it looks fine. A specific regression test is hard because the 
visual
artifacts manifest rarely and even so are not easy to test.
There should be plenty of time in JDK 15 for this to bake.

-phil.