Since few opengl contexts could be used at the same time,
CARenderer could use wrong context to render the video frame.

If the video surface contains an opengl context then need to use it as shared
regardless of existing of current context.

Change-Id: Ie7e87682c73c0fd341b40c05e3f3c4a70d563242
Reviewed-by: Andy Nichols <andy.nichols@qt.io>