better handle skipped frames and cache eviction

Add Renderer::skipFrame() which should be called when intentionally 
skipping frames, for instance because the screen content hasn't changed,
allowing Filament to performance needed periodic tasks, such as
cache garbage collection and callback dispatches.

We also improve the ResourceAllocator cache eviction policy:
- the cache is aggressively purged when skipping a frame
- we aggressively evict entries older than 3 frames

The default Config is now set to a more agressive setting.

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

@@ -28,6 +28,14 @@
 using namespace filament;
 using namespace backend;
 
+
+extern "C" JNIEXPORT void JNICALL
+Java_com_google_android_filament_Renderer_nSkipFrame(JNIEnv *, jclass, jlong nativeRenderer,
+        jlong vsyncSteadyClockTimeNano) {
+    Renderer *renderer = (Renderer *) nativeRenderer;
+    renderer->skipFrame(uint64_t(vsyncSteadyClockTimeNano));
+}
+
 extern "C" JNIEXPORT jboolean JNICALL
 Java_com_google_android_filament_Renderer_nBeginFrame(JNIEnv *, jclass, jlong nativeRenderer,
         jlong nativeSwapChain, jlong frameTimeNanos) {