"fullscreen" callback: do nothing if value is unchanged
The old video output core assumes that the fullscreen state must be toggled if the VOUT_FULLSCREEN_CHANGE bit is set. So we need to check that the target state is not already correct, as the VLC variables core does NOT do it internally. This commit provides rare exception to the rule that oldval is useless. This fixes a whole class of race conditions where two threads try to change fullscreen status at the same time. In the video filter case, we now enable fullscreen on all childrens, not just one. This seems a bit more logical for wall. Without this, toggling fullscreen would only ever work on the first video output. With this, things should work great if the different pieces of the wall are on different video ports, and OK (Alt+Tab is your friend) otherwise.
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