Add AgX tonemapper option to Environment

Technical implementation notes:

- Moved linearization step to before the outset matrix is applied and
  changed polynomial contrast curve approximation.
  - This does *not* implement Blender's chroma rotation to address hue shift.
    This hue rotation was found to have a significant performance impact.
- Improved performance by combining the AgX outset matrix with the Rec 2020 matrix.

Co-authored-by: Allen Pestaluky <allenpestaluky@gmail.com>
Co-authored-by: Clay John <claynjohn@gmail.com>

servers/rendering_server.h

@@ -1243,7 +1243,8 @@ public:
 		ENV_TONE_MAPPER_LINEAR,
 		ENV_TONE_MAPPER_REINHARD,
 		ENV_TONE_MAPPER_FILMIC,
-		ENV_TONE_MAPPER_ACES
+		ENV_TONE_MAPPER_ACES,
+		ENV_TONE_MAPPER_AGX,
 	};
 
 	virtual void environment_set_tonemap(RID p_env, EnvironmentToneMapper p_tone_mapper, float p_exposure, float p_white) = 0;