vdisp/sdl2: set correct data_len if pitch!=linelen

fixes:
```
uv -t testcard:codec=RGB:size=1921x1080 -d sdl
```

Doesn't crash but errors are printed on console and data at the end
(perhaps something like a line) is not written.

src/video_display/sdl2.c

@@ -468,6 +468,7 @@ static bool recreate_textures(struct state_sdl2 *s, struct video_desc desc) {
                 struct video_frame *f = vf_alloc_desc(desc);
                 f->callbacks.dispose_udata = (void *) texture;
                 SDL_CHECK(SDL_LockTexture(texture, NULL, (void **) &f->tiles[0].data, &s->texture_pitch));
+                f->tiles[0].data_len = desc.height * s->texture_pitch;
                 f->callbacks.data_deleter = vf_sdl_texture_data_deleter;
                 simple_linked_list_append(s->free_frame_queue, f);
         }