Mythgallery: Fix white screen after playing video when using OpenGL

If the mythfrontend UI is set to use OpenGL and MythGallery is set to use
OpenGL transitions then when video playback exits a white screen is
shown instead of the video screenshot with "Press SELECT to play again"
overlay.

The fix is to call makeCurrent on return from playback.

Signed-off-by: Lawrence Rust <lvr@softsystem.co.uk>
Signed-off-by: Jean-Yves Avenard <jyavenard@mythtv.org>