Check and fix indirect lighting power

[AlexandreLamure]

Indirect lighting seems too weak
Blender:
With "bright sky" (using background color as light)

Our results :

[AlexandreLamure]

It seems to come from the BRDF.
More specifically, the indirect version of the BRDF (eval_indirect()).

[AlexandreLamure]

There are also some weird artifacts in our version

The normals seem to flipped sometimes (maybe the NDF ?). Or it could be a bug in the metalness values

[AlexandreLamure]

Better examples (indirect light only, using background color as the light)

Blender:

Ours:

[AlexandreLamure]

Other tests:
1 - With point light
Blender:

Our version :

Our version with eval_direct only:

2 - With bright sky
Blender:

Our version :

Our version with eval_direct only:

[AlexandreLamure]

We think there is a problem in metalness

Blender:

Ours: