Functions which return function pointers now return SDL_FunctionPointer instead of void*

This fixes the clang warning "Cast between pointer-to-function and pointer-to-object is an extension"

You can define SDL_FUNCTION_POINTER_IS_VOID_POINTER in your project to restore the previous behavior.

Fixes https://github.com/libsdl-org/SDL/issues/2866

include/SDL3/SDL_loadso.h

@@ -89,8 +89,8 @@ extern DECLSPEC void *SDLCALL SDL_LoadObject(const char *sofile);
  * \sa SDL_LoadObject
  * \sa SDL_UnloadObject
  */
-extern DECLSPEC void *SDLCALL SDL_LoadFunction(void *handle,
-                                               const char *name);
+extern DECLSPEC SDL_FunctionPointer SDL_LoadFunction(void *handle,
+                                                     const char *name);
 
 /**
  * Unload a shared object from memory.