Convert unsigned long to uintptr_t for fixes compiling error on win32/mingw64

```
../src/vrend_renderer.c:5162:75: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
 5162 |                glDrawElementsInstancedBaseVertex(mode, info->count, elsz, (void *)(unsigned long)sub_ctx->ib.offset, info->instance_count, info->index_bias);
```

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
2 files changed