config: use correct constants

The value returned by config_GetType() is one of the VLC_VAR_*. The value of param->item.i_type is one of the CONFIG_ITEM_*.

The function config_GetType() both finds the vlc_param from a name and converts the type.

Since commit 82956e7b, the type is not converted anymore, so it must be compared against CONFIG_ITEM_* constants directly.

In practice, an internal vflip flag was not correct, so the OpenGL filter output was vflipped:

./vlc --video-filter=opengl --opengl-filter=mock video.mp4