Fix T77267: Render EEVEE AO pass when AO disabled.

In EEVEE the AO renderpass influenced other render passes. Until now the
pass wasn't selectable when AO was disabled in the scene to remove these
render artifacts.

This patch allows rendering EEVEE AO pass without enabling it in the
scene. It does this by binding a fallback texture that is used by the
surface shaders.

Reviewed By: Clément Foucault

Differential Revision: https://developer.blender.org/D7956

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

@@ -359,11 +359,6 @@ static void eevee_render_result_occlusion(RenderLayer *rl,
                                           EEVEE_Data *vedata,
                                           EEVEE_ViewLayerData *sldata)
 {
-  if ((vedata->stl->effects->enabled_effects & EFFECT_GTAO) == 0) {
-    /* AO is not enabled. */
-    return;
-  }
-
   if ((vedata->stl->g_data->render_passes & EEVEE_RENDER_PASS_AO) != 0) {
     EEVEE_renderpasses_postprocess(sldata, vedata, EEVEE_RENDER_PASS_AO);
     eevee_render_color_result(