gl_video: use hwdec download_image call only if hwdec is active

The hwdec driver can be loaded, even if it's not used (e.g. when playing
a file with no hardware decoding after one with it enabled).

Also, check whether dlimage is NULL. Since this will do call into the
native hwdec API, there's a chance a driver could fail doing this, it's
better to check the return value, even if this case currently can't
happen.
manpage: mention that vda can work with vo_opengl

Also, with the addition of VDA, all hardware decoding backends work with
vo_opengl now.