gltfio: Introduce the asynchronous API and use it.

We were already using jobs for decoding PNG and JPEG files, but we were
doing a join. This add three methods to ResourceLoader that allow
clients to amortize the decoding process across multiple frames, even on
single-threaded platforms like WebGL.

This PR adds async loading to the following demos:
- samples/gltf_viewer (now shows a progress bar in the UI)
- android/sample-gltf-viewer
- web/samples/helmet.html

Fixes #1876.

samples/gltf_viewer.cpp

@@ -53,6 +53,7 @@ struct App {
     MaterialProvider* materials;
     MaterialSource materialSource = GENERATE_SHADERS;
     bool actualSize = false;
+    gltfio::ResourceLoader* resourceLoader = nullptr;
 };
 
 static const char* DEFAULT_IBL = "venetian_crossroads_2k";
@@ -199,15 +200,15 @@ int main(int argc, char** argv) {
         configuration.gltfPath = filename.getAbsolutePath();
         configuration.normalizeSkinningWeights = true;
         configuration.recomputeBoundingBoxes = false;
-        gltfio::ResourceLoader(configuration).loadResources(app.asset);
+        if (!app.resourceLoader) {
+            app.resourceLoader = new gltfio::ResourceLoader(configuration);
+        }
+        app.resourceLoader->asyncBeginLoad(app.asset);
 
         // Load animation data then free the source hierarchy.
         app.asset->getAnimator();
         app.asset->releaseSourceData();
 
-        // Add the renderables to the scene.
-        app.viewer->setAsset(app.asset, !app.actualSize);
-
         auto ibl = FilamentApp::get().getIBL();
         if (ibl) {
             app.viewer->setIndirectLight(ibl->getIndirectLight(), ibl->getSphericalHarmonics());
@@ -231,6 +232,10 @@ int main(int argc, char** argv) {
         loadResources(filename);
 
         app.viewer->setUiCallback([&app, scene] () {
+            float progress = app.resourceLoader->asyncGetLoadProgress();
+            if (progress < 1.0) {
+                ImGui::ProgressBar(progress);
+            }
             if (ImGui::CollapsingHeader("Stats")) {
                 ImGui::Text("%zu entities in the asset", app.asset->getEntityCount());
                 ImGui::Text("%zu renderables (excluding UI)", scene->getRenderableCount());
@@ -253,6 +258,13 @@ int main(int argc, char** argv) {
     };
 
     auto animate = [&app](Engine* engine, View* view, double now) {
+        app.resourceLoader->asyncUpdateLoad();
+
+        // Add the renderables to the scene after the textures have finished loading.
+        if (app.resourceLoader->asyncGetLoadProgress() == 1.0f) {
+            app.viewer->setAsset(app.asset, !app.actualSize);
+        }
+
         app.viewer->applyAnimation(now);
     };