Add Engine::Print method

Also add a hello world example to readme

README.md

@@ -27,3 +27,43 @@ cd build/android
 [vma](https://github.com/GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator),
 [vulkan-sdk](https://vulkan.lunarg.com),
 [volk](https://github.com/zeux/volk)
+
+#### Hello World example:
+Simply shows a rotating "Hello Wolrd!".
+```cpp
+#include "base/interpolation.h"
+#include "engine/animator.h"
+#include "engine/asset/image.h"
+#include "engine/engine.h"
+#include "engine/game.h"
+#include "engine/game_factory.h"
+#include "engine/image_quad.h"
+
+class HelloWorld final : public eng::Game {
+ public:
+  ~HelloWorld() final = default;
+
+  bool Initialize() final {
+    eng::Engine::Get().SetImageSource(
+        "hello_world_image",
+        std::bind(&eng::Engine::Print, &eng::Engine::Get(), "Hello World!",
+                  base::Vector4f(1, 1, 1, 0)));
+
+    hello_world_.Create("hello_world_image");
+    hello_world_.SetVisible(true);
+    animator_.Attach(&hello_world_);
+    animator_.SetRotation(base::PI2f, 3,
+                          std::bind(base::SmootherStep, std::placeholders::_1));
+    animator_.Play(eng::Animator::kRotation, true);
+    return true;
+  }
+
+ private:
+  eng::ImageQuad hello_world_;
+  eng::Animator animator_;
+};
+
+DECLARE_GAME_BEGIN
+DECLARE_GAME(HelloWorld)
+DECLARE_GAME_END
+```

src/engine/engine.cc

@@ -442,6 +442,19 @@ TextureCompressor* Engine::GetTextureCompressor(bool opacity) {
   return opacity ? tex_comp_alpha_.get() : tex_comp_opaque_.get();
 }
 
+std::unique_ptr<Image> Engine::Print(const std::string& text,
+                                     Vector4f bg_color) {
+  int width, height;
+  system_font_->CalculateBoundingBox(text.c_str(), width, height);
+
+  auto image = std::make_unique<Image>();
+  image->Create(width, height);
+  image->Clear(bg_color);
+  system_font_->Print(0, 0, text.c_str(), image->GetBuffer(),
+                      image->GetWidth());
+  return image;
+}
+
 int Engine::GetScreenWidth() const {
   return renderer_->GetScreenWidth();
 }

src/engine/engine.h

@@ -112,6 +112,9 @@ class Engine : public PlatformObserver {
 
   const Font* GetSystemFont() { return system_font_.get(); }
 
+  std::unique_ptr<Image> Print(const std::string& text,
+                               base::Vector4f bg_color);
+
   base::Randomf& GetRandomGenerator() { return random_; }
 
   TextureCompressor* GetTextureCompressor(bool opacity);

src/engine/game.h

@@ -9,18 +9,18 @@ class Game {
   virtual ~Game() = default;
 
   // Called before async-loading assets.
-  virtual bool PreInitialize() = 0;
+  virtual bool PreInitialize() { return true; }
 
   // Called after resources are created.
-  virtual bool Initialize() = 0;
+  virtual bool Initialize() { return true; }
 
-  virtual void Update(float delta_time) = 0;
+  virtual void Update(float delta_time) {}
 
-  virtual void ContextLost() = 0;
+  virtual void ContextLost() {}
 
-  virtual void LostFocus() = 0;
+  virtual void LostFocus() {}
 
-  virtual void GainedFocus(bool from_interstitial_ad) = 0;
+  virtual void GainedFocus(bool from_interstitial_ad) {}
 
  private:
   Game(const Game&) = delete;