Public API for PointLight attenuation

[Nir Lisker]

Hi,

This API will be used as the CSR for JDK-8217472
<https://bugs.openjdk.java.net/browse/JDK-8217472>. It is posted on GitHub:

https://github.com/javafxports/openjdk-jfx/issues/256#issuecomment-459499308

Any comments are welcome.

Thanks,
Nir