Frontends: Add shared command line interface

Also provides batch mode and automatic fullscreen switching.

  -help: Displays this information and exits.
  -version: Displays version information and exits.
  -batch: Enables batch mode (exits after powering off)
  -fastboot: Force fast boot for provided filename
  -slowboot: Force slow boot for provided filename
  -resume: Load resume save state. If a boot filename is provided,
    that game's resume state will be loaded, otherwise the most
    recent resume save state will be loaded.
  -state <index>: Loads specified save state by index. If a boot
    filename is provided, a per-game state will be loaded, otherwise
    a global state will be loaded.
  -statefile <filename>: Loads state from the specified filename.
    No boot filename is required with this option.
  -fullscreen: Enters fullscreen mode immediately after starting.
  -nofullscreen: Prevents fullscreen mode from triggering if enabled.
  -portable: Forces "portable mode", data in same directory.
  --: Signals that no more arguments will follow and the remaining
    parameters make up the filename. Use when the filename contains
    spaces or starts with a dash.

src/duckstation-sdl/sdl_host_interface.cpp

@@ -34,6 +34,11 @@ SDLHostInterface::SDLHostInterface()
 
 SDLHostInterface::~SDLHostInterface() = default;
 
+const char* SDLHostInterface::GetFrontendName() const
+{
+  return "DuckStation SDL/ImGui Frontend";
+}
+
 float SDLHostInterface::GetDPIScaleFactor(SDL_Window* window)
 {
 #ifdef __APPLE__
@@ -191,10 +196,6 @@ bool SDLHostInterface::AcquireHostDisplay()
   }
 #endif
 
-  // Switch to fullscreen if requested.
-  if (m_settings.start_fullscreen)
-    SetFullscreen(true);
-
   return true;
 }
 
@@ -274,6 +275,11 @@ void SDLHostInterface::OnRunningGameChanged()
     SDL_SetWindowTitle(m_window, "DuckStation");
 }
 
+void SDLHostInterface::RequestExit()
+{
+  m_quit_request = true;
+}
+
 void SDLHostInterface::RunLater(std::function<void()> callback)
 {
   SDL_Event ev = {};