Multi-thread shaders generation when compiling materials (#3464)

* Stop using members as globals between methods

* Multi-thread shaders generation with JobSystem

* Pass JobSystem to MaterialBuilder::build()

* Fix MeshAssimp to use the new API

* Allow the Java API to pass a job system via Engine

* Update docs

* Apply suggestions from code review

Co-authored-by: Philip Rideout <philiprideout@gmail.com>

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

@@ -306,3 +306,9 @@ Java_com_google_android_filament_Engine_nGetRenderableManager(JNIEnv*, jclass,
     Engine* engine = (Engine*) nativeEngine;
     return (jlong) &engine->getRenderableManager();
 }
+
+extern "C" JNIEXPORT jlong JNICALL
+Java_com_google_android_filament_Engine_nGetJobSystem(JNIEnv*, jclass, jlong nativeEngine) {
+    Engine* engine = (Engine*) nativeEngine;
+    return (jlong) &engine->getJobSystem();
+}