Replaced implicit emissive weight with default. (#13871)

Since `StandardMaterial::emissive_exposure_weight` does not get packed
into the gbuffer in the deferred case, unpacking uses an implicit
default value for emissive's alpha channel.

This resulted in divergent behavior between the forward and deferred
renderers when using standard materials with default
emissive_exposure_weight, this value defaulting to `0.0` in the forward
case and `1.0` in the other.

This patch changes the implicit value in the deferred case to `0.0` in
order to match the behavior of the forward renderer. However, this still
does not solve the case where `emissive_exposure_weight` is not `0.0`.

Author: Dragoș Tiselice

crates/bevy_pbr/src/deferred/pbr_deferred_functions.wgsl

@@ -76,10 +76,10 @@ fn pbr_input_from_deferred_gbuffer(frag_coord: vec4<f32>, gbuffer: vec4<u32>) ->
     let emissive = rgb9e5::rgb9e5_to_vec3_(gbuffer.g);
     if ((pbr.material.flags & STANDARD_MATERIAL_FLAGS_UNLIT_BIT) != 0u) {
         pbr.material.base_color = vec4(emissive, 1.0);
-        pbr.material.emissive = vec4(vec3(0.0), 1.0);
+        pbr.material.emissive = vec4(vec3(0.0), 0.0);
     } else {
         pbr.material.base_color = vec4(pow(base_rough.rgb, vec3(2.2)), 1.0);
-        pbr.material.emissive = vec4(emissive, 1.0);
+        pbr.material.emissive = vec4(emissive, 0.0);
     }
 #ifdef WEBGL2 // More crunched for webgl so we can also fit depth.
     let props = deferred_types::unpack_unorm3x4_plus_unorm_20_(gbuffer.b);