Implement blurring of VSM shadow maps

This is still somewhat work-in-progress, but good enough to be useful.

Some caveats:
- large blurs don't work well with cascades
- the implementation uses more memory than it could

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

@@ -77,7 +77,7 @@ Java_com_google_android_filament_LightManager_nBuilderShadowOptions(JNIEnv* env,
         jlong nativeBuilder, jint mapSize, jint cascades, jfloatArray splitPositions,
         jfloat constantBias, jfloat normalBias, jfloat shadowFar, jfloat shadowNearHint,
         jfloat shadowFarHint, jboolean stable, jboolean screenSpaceContactShadows, jint stepCount,
-        jfloat maxShadowDistance, jint vsmMsaaSamples) {
+        jfloat maxShadowDistance, jint vsmMsaaSamples, jfloat blurStandardDeviation) {
     LightManager::Builder *builder = (LightManager::Builder *) nativeBuilder;
     LightManager::ShadowOptions shadowOptions {
             .mapSize = (uint32_t)mapSize,
@@ -92,7 +92,8 @@ Java_com_google_android_filament_LightManager_nBuilderShadowOptions(JNIEnv* env,
             .stepCount = uint8_t(stepCount),
             .maxShadowDistance = maxShadowDistance,
             .vsm = {
-                    .msaaSamples = (uint8_t) vsmMsaaSamples
+                    .msaaSamples = (uint8_t) vsmMsaaSamples,
+                    .blurStandardDeviation = blurStandardDeviation
             }
     };
     jfloat *nativeSplits = env->GetFloatArrayElements(splitPositions, NULL);