We now use WebP Demux API to decode both single image format and muxed animation format.

Change-Id: Ia2922892a3a626e9921c3910801d7c975d9fc6a2
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>