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/jni_bridge.cpp

@@ -102,6 +102,11 @@ Java_com_lazy_emulate_emulation_NativeLibretro_nativeSetFrameSkip(JNIEnv*, jobje
     LibretroFrontend::instance().setFrameSkip(skip);
 }
 
+JNIEXPORT void JNICALL
+Java_com_lazy_emulate_emulation_NativeLibretro_nativeSetDisplayMode(JNIEnv*, jobject, jint mode) {
+    LibretroFrontend::instance().setDisplayMode(mode);
+}
+
 JNIEXPORT void JNICALL
 Java_com_lazy_emulate_emulation_NativeLibretro_nativeSetSystemDir(JNIEnv* env, jobject, jstring dir) {
     const char* d = env->GetStringUTFChars(dir, nullptr);