RFR: 8234921: Add DirectionalLight to the selection of 3D light types

[Kevin Rushforth]

On Sat, 14 Aug 2021 12:58:01 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> Adds a directional light as a subclass of `LightBase`. I think that this is the correct hierarchy for it.
>> 
>> I tried to simulate a directional light by putting a point light far away, but I got artifacts when the distance was large. Instead, I added an on/off attenuation flag as the 4th component of the attenuation 4-vector. When it is 0, a simpler computation is used in the pixel/fragment shader that calculates the illumination based on the light direction only (making the position variables meaningless). When it is 1, the point/spot light computation is used. It's possible that the vertex shader can also be simplified in this case since it does not need to transform the position vectors, but I left this optimization avenue for another time.
>> 
>> I noticed a drop of ~1 fps in the stress test of 5000 meshes.
>> 
>> I added a system test that verifies the correct color result from a few directions. I also updated the lighting sample application to include 3 directional lights and tested them on all the models visually. The lights seem to behave the way I would expect.
>
> My commit message was wrong. I updated the `@since` tag, not the copyright year. I'll let the copyright year script do all the changed files together instead of doing it by hand.

@nlisker I reviewed the CSR as well, so you can finalize it.

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PR: https://git.openjdk.java.net/jfx/pull/548