Fix preloading of video when using QtMultimedia backend.

Based on a patch by Yoann Lopes (yoann.lopes@theqtcompany.com).

Task-number: QTBUG-37381
Change-Id: I8647c578399a592f8feef0d4028187afd3726b2d
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>