videonode: imx6: Ensure the video node excludes padding pixels

videonode: imx6: Ensure the video node excludes padding pixels

Padding rows & columns are included in the video frame that is sent to
the VIV direct texture. Scale the UV coordinates to ensure the padding
pixels aren't shown.

Change-Id: I8b870a95ff786f9f80f42f0cc33f468b0c9c3863
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>