Add display mode options: stretch, aspect ratio, and integer scaling

Renders the emulator framebuffer into a calculated sub-rect with
nearest-neighbor scaling and black bars, driven by the core's
reported aspect ratio. Adds a cycle button in the game HUD and
a display mode setting in the settings screen.

app/src/main/cpp/libretro_frontend.h

@@ -10,6 +10,12 @@
 #include <thread>
 #include <unordered_map>
 
+enum class DisplayMode : int {
+    STRETCH = 0,
+    ASPECT_RATIO = 1,
+    INTEGER_SCALE = 2
+};
+
 class LibretroFrontend {
 public:
     static LibretroFrontend& instance();
@@ -33,6 +39,7 @@ public:
 
     void setAudioEnabled(bool enabled);
     void setFrameSkip(int skip);
+    void setDisplayMode(int mode);
 
     void setSystemDir(const char* dir);
     void setSaveDir(const char* dir);
@@ -90,6 +97,10 @@ private:
     ANativeWindow* window_ = nullptr;
     std::mutex window_mutex_;
     unsigned pixel_format_ = RETRO_PIXEL_FORMAT_RGB565;
+    std::atomic<DisplayMode> display_mode_{DisplayMode::STRETCH};
+    float core_aspect_ratio_ = 0.0f;
+    unsigned core_base_width_ = 0;
+    unsigned core_base_height_ = 0;
 
     // Audio
     AudioEngine audio_engine_;