Eevee: Rework GTAO

This includes big improvement:
- The horizon search is decoupled from the BSDF evaluation. This means using multiple BSDF nodes have a much lower impact when enbaling AO.
- The horizon search is optimized by splitting the search into 4 corners searching similar directions to help which GPU cache coherence.
- The AO options are now uniforms and do not trigger shader recompilation (aka. freeze UI).
- Include a quality slider similar to the SSR one.
- Add a switch for disabling bounce light approximation.
- Fix problem with Bent Normals when occlusion get very dark.
- Add a denoise option to that takes the neighbors pixel values via glsl derivatives. This reduces noise but exhibit 2x2 blocky artifacts.

The downside : Separating the horizon search uses more memory (~3MB for each samples on HD viewport). We could lower the bit depth to 4bit per horizon but it produce noticeable banding (might be fixed with some dithering).

source/blender/draw/engines/eevee/eevee_lightprobes.c

@@ -969,8 +969,7 @@ static void render_scene_to_probe(
 	/* Disable AO until we find a way to hide really bad discontinuities between cubefaces. */
 	tmp_ao_dist = stl->effects->ao_dist;
 	tmp_ao_samples = stl->effects->ao_samples;
-	stl->effects->ao_dist = 0.0f;
-	stl->effects->ao_samples = 0.0f;
+	stl->effects->ao_settings = 0.0f; /* Disable AO */
 
 	/* 1 - Render to each cubeface individually.
 	 * We do this instead of using geometry shader because a) it's faster,
@@ -1099,6 +1098,9 @@ static void render_scene_to_planar(
 
 	EEVEE_create_minmax_buffer(vedata, tmp_planar_depth, layer);
 
+	/* Compute GTAO Horizons */
+	EEVEE_effects_do_gtao(sldata, vedata);
+
 	/* Rebind Planar FB */
 	DRW_framebuffer_bind(fbl->planarref_fb);