tray: document thread-safety

src/tray/cocoa/SDL_tray.m

@@ -80,6 +80,11 @@ static void DestroySDLMenu(SDL_TrayMenu *menu)
 
 SDL_Tray *SDL_CreateTray(SDL_Surface *icon, const char *tooltip)
 {
+    if (!SDL_IsMainThread()) {
+        SDL_SetError("This function should be called on the main thread");
+        return NULL;
+    }
+
     if (icon) {
         icon = SDL_ConvertSurface(icon, SDL_PIXELFORMAT_RGBA32);
         if (!icon) {

src/tray/unix/SDL_tray.c

@@ -398,6 +398,11 @@ static void DestroySDLMenu(SDL_TrayMenu *menu)
 
 SDL_Tray *SDL_CreateTray(SDL_Surface *icon, const char *tooltip)
 {
+    if (!SDL_IsMainThread()) {
+        SDL_SetError("This function should be called on the main thread");
+        return NULL;
+    }
+
     if (init_gtk() != true) {
         return NULL;
     }

src/tray/windows/SDL_tray.c

@@ -211,6 +211,11 @@ static HICON load_default_icon()
 
 SDL_Tray *SDL_CreateTray(SDL_Surface *icon, const char *tooltip)
 {
+    if (!SDL_IsMainThread()) {
+        SDL_SetError("This function should be called on the main thread");
+        return NULL;
+    }
+
     SDL_Tray *tray = (SDL_Tray *)SDL_calloc(1, sizeof(*tray));
 
     if (!tray) {