Discourage using || on is_specular

[trevordblack]

"Note that due to WGSL's short-circuiting rules, the call to schlick_fresnel won't evaluate if cannot_refract is true, skipping the computation at angles of total internal reflection. "

This is perfectly fine and will work forever

But this line is doing a lot

    is_specular = cannot_refract || schlick_fresnel(ref_ratio, cos_theta) > rand_f32();

consider splitting into multiple if statements.