SSR: add the useFresnel property to generate more physically accurate results

[Popov72]

Extract from the useFresnel property doc:

Gets or sets a boolean indicating whether the blending between the current color pixel and the reflection color should be done with a Fresnel coefficient (default: false).
It is more physically accurate to use the Fresnel coefficient (otherwise it uses the reflectivity of the material for blending), but it is also more expensive when you use blur (when blurDispersionStrength > 0).

I also disable the reflectivity test when the threshold is 0, to save some branching in the shader code.

I'll update the documentation once the PR has been merged.

[bjsplat]

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[bjsplat]

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[sebavan]

Just a tiny NEAT and good to go