Some third party themes have tab body part defined but with its ImageSelectType set to IST_NONE.
In this case, DrawThemeBackground() will fail to draw the background.

Fix track bar black background in winecfg when using VLT theme.
Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>