[OpenJDK 2D-Dev] RFR 8199789: Emit a warning message when t2k is selected via system property

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8199789
Webrev : http://cr.openjdk.java.net/~prr/8199789/

https://bugs.openjdk.java.net/browse/JDK-8193017 made
freetype the default font rasteriser for all JDK builds.

We plan to remove t2k completely including references to it
from open sources, before JDK 11 GA's

But for now it is still there for debugging but to make it clear
this small fix makes a warning get printed if you try to use it.

It also adds a new value for sun.java2d.font.scaler of "legacy" which
means "t2k" but as it was used in by default.

The subtle issue is that "t2k" disables using GDI for LCD text.
"legacy" gets you exactly what JDK did by default in 6u10 -> JDK 10 
inclusive.

So it may be more useful for a debugging comparison flag.

-phil.