message: RenderTarget demo should create depth attachment.
  This "fixes" which was not really a bug.  (doh)
diff: @@ -64,7 +64,8 @@ struct App {
  utils::Entity reflectedMonkey;
  mat4f transform;
  - Texture* offscreenTexture = nullptr;
  + Texture* offscreenColorTexture = nullptr;
  + Texture* offscreenDepthTexture = nullptr;
  RenderTarget* offscreenRenderTarget = nullptr;
  View* offscreenView = nullptr;
  Scene* offscreenScene = nullptr;
  @@ -201,12 +202,17 @@ int main(int argc, char** argv) {
  app.offscreenScene = engine->createScene();
  app.offscreenView->setScene(app.offscreenScene);
  app.offscreenView->setPostProcessingEnabled(false);
  - app.offscreenTexture = Texture::Builder()
  + app.offscreenColorTexture = Texture::Builder()
  .width(vp.width).height(vp.height).levels(1)
  .usage(Texture::Usage::COLOR_ATTACHMENT | Texture::Usage::SAMPLEABLE)
  .format(Texture::InternalFormat::RGBA8).build(*engine);
  + app.offscreenDepthTexture = Texture::Builder()
  + .width(vp.width).height(vp.height).levels(1)
  + .usage(Texture::Usage::DEPTH_ATTACHMENT)
  + .format(Texture::InternalFormat::DEPTH24).build(*engine);
  app.offscreenRenderTarget = RenderTarget::Builder()
  - .texture(RenderTarget::COLOR, app.offscreenTexture)
  + .texture(RenderTarget::COLOR, app.offscreenColorTexture)
  + .texture(RenderTarget::DEPTH, app.offscreenDepthTexture)
  .build(*engine);
  app.offscreenView->setRenderTarget(app.offscreenRenderTarget);
  app.offscreenView->setViewport({0, 0, vp.width, vp.height});
  @@ -252,7 +258,7 @@ int main(int argc, char** argv) {
  .build(*engine);
  app.quadMatInstance = app.quadMaterial->createInstance();
  TextureSampler sampler(TextureSampler::MinFilter::LINEAR, TextureSampler::MagFilter::LINEAR);
  - app.quadMatInstance->setParameter("albedo", app.offscreenTexture, sampler);
  + app.quadMatInstance->setParameter("albedo", app.offscreenColorTexture, sampler);
  app.quadEntity = em.create();
  RenderableManager::Builder(1)
  .boundingBox({{ -1, -1, -1 }, { 1, 1, 1 }})
  @@ -320,7 +326,8 @@ int main(int argc, char** argv) {
  engine->destroy(app.monkeyMesh.renderable);
  engine->destroy(app.monkeyMesh.vertexBuffer);
  engine->destroy(app.monkeyMesh.indexBuffer);
  - engine->destroy(app.offscreenTexture);
  + engine->destroy(app.offscreenColorTexture);
  + engine->destroy(app.offscreenDepthTexture);
  engine->destroy(app.offscreenRenderTarget);
  engine->destroy(app.offscreenScene);
  engine->destroy(app.offscreenView);
  