Add spatial light shader built in for light_specular property

[JohanAR]

Light3D has a light_specular property which is used to set the intensity of specular contributed by this light source, but it was previously only used by the default material light shader, and not possible to use in a custom light() shader.

[JohanAR]

I don't think it's ideal to have both LIGHT_SPECULAR (this) and SPECULAR_LIGHT (output from light shader), but there are some arguments for it IMO:

• The property on Light3D is named "light_specular", although if I read the code correctly, specular_amount is actually 2.0 * light_specular so it's not exactly the same.
• I think it makes it more clear that this specular value comes from the light source and it's not the specular value set on a StandardMaterial3D.
• It matches LIGHT_COLOR, which also comes from a Light3D property.

I did not try to change the gles3 renderer because I couldn't get omni or spot lights working in a custom light shader there, they did not appear to contribute any light at all.

[clayjohn]

This seems very helpful!

2 things:

1. Changes should be made for the OpenGL renderer as well
2. While I agree with you that LIGHT_SPECULAR would be the ideal name, the similarity to SPECULAR_LIGHT is just too close to be ignored. Unfortunately, I think that SPECULAR_AMOUNT would be a better name.

[JohanAR]

Thanks for the review. Updated according to suggestions.

Now I got omni lights working in the gles3 renderer without issues, dunno why it didn't last time.

[JohanAR]

Docs update godotengine/godot-docs#7277

[clayjohn]

Looks good!

[clayjohn]

Note to whoever merges this, the Docs PR can be merged right after. godotengine/godot-docs#7277

[mhilbrunner]

Merged, merged the docs PR as well - thanks for contributing!