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>

samples/sample_normal_map.cpp

@@ -299,7 +299,7 @@ static void setup(Engine* engine, View*, Scene* scene) {
             .parameter(MaterialBuilder::SamplerType::SAMPLER_2D, "baseColorMap");
     }
 
-    Package pkg = builder.build();
+    Package pkg = builder.build(engine->getJobSystem());
 
     g_material = Material::Builder().package(pkg.getData(), pkg.getSize())
             .build(*engine);