System: Disable screensaver inhibit on Gamescope

So I don't have a barrage of Steam Deck users whinging about the OSD
message.

Also disable it if XDG_CURRENT_DESKTOP is missing, like running directly
on the framebuffer.

src/core/system.cpp

@@ -319,6 +319,9 @@ struct ALIGN_TO_CACHE_LINE StateVars
   GameHash running_game_hash = 0;
   bool running_game_custom_title = false;
 
+  // Prevent screensaver inhibits when running on platforms that don't have it (e.g. gamescope).
+  bool disable_screensaver_inhibit = false;
+
   std::atomic_bool startup_cancelled{false};
 
   std::unique_ptr<INISettingsInterface> game_settings_interface;
@@ -503,11 +506,19 @@ bool System::ProcessStartup(Error* error)
   SetRymlCallbacks();
 
 #ifdef __linux__
-  // Running DuckStation out of /usr/lib is not supported and makes no sense.
-  if (std::memcmp(EmuFolders::AppRoot.data(),
-                  "/usr/"
-                  "lib",
-                  8) == 0)
+  // Disable screensaver inhibit if running on gamescope.
+  const char* desktop = std::getenv("XDG_CURRENT_DESKTOP");
+  if (!desktop)
+    desktop = std::getenv("XDG_SESSION_DESKTOP");
+  if (!desktop || std::strlen(desktop) == 0 || std::strstr(desktop, "gamescope"))
+  {
+    INFO_LOG("Missing XDG_CURRENT_DESKTOP ({}) or running under gamescope, disabling screensaver inhibit.",
+             desktop ? desktop : "null");
+    s_state.disable_screensaver_inhibit = true;
+  }
+
+  // Running DuckStation out of /usr is not supported and makes no sense.
+  if (std::memcmp(EmuFolders::AppRoot.data(), "/usr/", 5) == 0)
     return false;
 #endif
 
@@ -3848,6 +3859,9 @@ bool System::ShouldAllowPresentThrottle()
 
 void System::InhibitScreensaver(bool inhibit)
 {
+  if (s_state.disable_screensaver_inhibit)
+    return;
+
   Error error;
   if (!Host::SetScreensaverInhibit(inhibit, &error))
   {