[OpenJDK 2D-Dev] RFR: 8218077: Move src/java.desktop/share/legal/mesa3d.md to unix location

[Philip Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8218077
Webrev : http://cr.openjdk.java.net/~prr/8218077/

This fix does not change the file's contents, it just moves
it to the unix specific location to match the usage.

-phil.