More improvements to SSAO

- we add an 'intensity' parameter that allows to control the strength
  of the AO effect. This is useful for aesthetic reasons.

- the default intensity is now 2x that of before this change, which
  makes the intensity parameter match AO papers this implementation
  is inspired of.

- bias is now z-dependent, which reduces some self shadowing wrt the
  depth.

android/filament-android/src/main/cpp/View.cpp

@@ -241,8 +241,14 @@ Java_com_google_android_filament_View_nGetAmbientOcclusion(JNIEnv*, jclass, jlon
 
 extern "C" JNIEXPORT void JNICALL
 Java_com_google_android_filament_View_nSetAmbientOcclusionOptions(JNIEnv*, jclass,
-    jlong nativeView, jfloat radius, jfloat bias, jfloat power, jfloat resolution) {
+    jlong nativeView, jfloat radius, jfloat bias, jfloat power, jfloat resolution, jfloat intensity) {
     View* view = (View*) nativeView;
-    View::AmbientOcclusionOptions options = { .radius = radius, .bias = bias, .power = power, .resolution = resolution};
+    View::AmbientOcclusionOptions options = {
+            .radius = radius,
+            .bias = bias,
+            .power = power,
+            .resolution = resolution,
+            .intensity = intensity
+    };
     view->setAmbientOcclusionOptions(options);
 }