SDL_gtk: Prevent loading GTK-3 if a different version of GTK is already loaded

- gtk_init_check claims it will not terminate the program on failure, however this is does
  not apply to the internal check it performs to see whether another GTK library is already
  loaded, the detection of which triggers a g_error which glib treats as fatal.

- gtk_progress_get_type and gtk_misc_get_type are checked to match the checks done
  by _gtk_module_has_mixed_deps.

- Clean up gtk/gdk loading. There are no library fallbacks so the one option available can
  be loaded rather than iterating through a list of one item.

src/core/unix/SDL_gtk.c

@@ -21,6 +21,8 @@
 #include "SDL_internal.h"
 #include "SDL_gtk.h"
 
+#include <dlfcn.h>
+
 #define SDL_GTK_SYM2_OPTIONAL(ctx, lib, sub, fn, sym)                     \
     ctx.sub.fn = (void *)SDL_LoadFunction(lib, #sym)
 
@@ -35,25 +37,19 @@
 #define SDL_GTK_SYM(ctx, lib, sub, fn) \
     SDL_GTK_SYM2(ctx, lib, sub, fn, sub##_##fn)
 
-// we never link directly to gtk
-static const char *gdk_names[] = {
 #ifdef SDL_PLATFORM_OPENBSD
-    "libgdk-3.so",
+#define GDK3_LIB "libgdk-3.so"
 #else
-    "libgdk-3.so.0",
+#define GDK3_LIB "libgdk-3.so.0"
 #endif
-    NULL
-};
 
-static const char *gtk_names[] = {
 #ifdef SDL_PLATFORM_OPENBSD
-    "libgtk-3.so",
+#define GTK3_LIB "libgtk-3.so"
 #else
-    "libgtk-3.so.0",
+#define GTK3_LIB "libgtk-3.so.0"
 #endif
-    NULL
-};
 
+// we never link directly to gtk
 static void *libgdk = NULL;
 static void *libgtk = NULL;
 
@@ -74,18 +70,6 @@ static void QuitGtk(void)
     libgtk = NULL;
 }
 
-static void *FindLib(const char **names)
-{
-    const char **name_ptr = names;
-    void *handle = NULL;
-
-    do {
-        handle = SDL_LoadObject(*name_ptr);
-    } while (*++name_ptr && !handle);
-
-    return handle;
-}
-
 static bool IsGtkInit()
 {
     return libgdk != NULL && libgtk != NULL;
@@ -101,8 +85,22 @@ static bool InitGtk(void)
         return true;
     }
 
-    libgdk = FindLib(gdk_names);
-    libgtk = FindLib(gtk_names);
+    // GTK only allows a single version to be loaded into a process at a time,
+    // so if there is one already loaded ensure it is the version we use.
+    void *progress_get_type = dlsym(RTLD_DEFAULT, "gtk_progress_get_type");
+    void *misc_get_type = dlsym(RTLD_DEFAULT, "gtk_misc_get_type");
+    if (progress_get_type || misc_get_type) {
+        void *libgtk3 = dlopen(GTK3_LIB, RTLD_NOLOAD | RTLD_LAZY);
+        if (!libgtk3) {
+            QuitGtk();
+            return SDL_SetError("Could not load GTK-3, another GTK version already present");
+        }
+
+        dlclose(libgtk3);
+    }
+
+    libgdk = SDL_LoadObject(GDK3_LIB);
+    libgtk = SDL_LoadObject(GTK3_LIB);
 
     if (!libgdk || !libgtk) {
         QuitGtk();