Fix Presentation Mode VideoWidget sizing

This fix smells a lot like a workaround for a bug in Qt but i could not
write down a sufficiently small testcase for it to report it to Qt, and
this is not wrong anyway since we eventually add playerPage to a
layout for this. What I tracked down was that since playerPage had no
parent, at some point a nativeId for it was being created and that
broke sizing back to the default 100x30 used in Qt instead of the
size we had given it.

For some reason i needed a PDF file with more than one video to
reproduce this problem, the first video was correctly sized but the
second one was 100x30.