Fix: EEVEE-Next: Firefly caused by denoising shiny surfaces

This was caused by float imprecision in the NDF computation when
alpha was very close to 0. The alpha clamping threshold for the
eval function needs to be higher than the one for the sampling
function.

Fix #118997

source/blender/draw/engines/eevee_next/shaders/eevee_bxdf_microfacet_lib.glsl

@@ -138,7 +138,10 @@ BsdfSample bxdf_ggx_sample_transmission(
 /* Compute the GGX BxDF without the Fresnel term, multiplied by the cosine foreshortening term. */
 BsdfEval bxdf_ggx_eval(vec3 N, vec3 L, vec3 V, float alpha, float eta, const bool do_reflection)
 {
-  alpha = max(square(BSDF_ROUGHNESS_THRESHOLD), alpha);
+  /* This threshold was computed based on precision of NVidia compiler (see #118997).
+   * These drivers tend to produce NaNs in the computation of the NDF (`D`) if alpha is close to 0.
+   */
+  alpha = max(1e-3, alpha);
 
   float LV = dot(L, V);
   float NV = dot(N, V);