Fix transmission by setting the correct value for transmissive_lighting_input.F_ab

[geckoxx]

Objective

• The clearcoat PR Implement clearcoat per the Filament and the KHR_materials_clearcoat specifications. #13031 had a small typo which broke transmission
• Fixes transmission renders black on some GPU #13284

Solution

• Set transmissive_lighting_input.F_ab to the correct value

[JMS55]

Approved, but I'm not sure where 0.1 came from for F_ab. @pcwalton probably knows, it would be nice to document it.

[mockersf]

works for me!
F_ab is also mentioned in several other place as having a 0.1 value for transmitted light contribution

bevy/crates/bevy_pbr/src/render/pbr_functions.wgsl

Lines 342 to 350 in d17fb16

	// NOTE: We use the diffuse transmissive color, the second Lambertian lobe's calculated
	// world position, inverted normal and view vectors, and the following simplified
	// values for a fully diffuse transmitted light contribution approximation:
	//
	// roughness = 1.0;
	// NdotV = 1.0;
	// R = vec3<f32>(0.0) // doesn't really matter
	// F_ab = vec2<f32>(0.1)
	// F0 = vec3<f32>(0.0)

[pcwalton]

Aha, sorry about that!