Fixes #2296 - SDL_GL_UnbindTexture segfaults (thanks Daniel B?nzli)

src/render/SDL_render.c

@@ -1876,7 +1876,9 @@ int SDL_GL_UnbindTexture(SDL_Texture *texture)
 
     CHECK_TEXTURE_MAGIC(texture, -1);
     renderer = texture->renderer;
-    if (renderer && renderer->GL_UnbindTexture) {
+    if (texture->native) {
+        return SDL_GL_UnbindTexture(texture->native);
+    } else if (renderer && renderer->GL_UnbindTexture) {
         return renderer->GL_UnbindTexture(renderer, texture);
     }