vout: do not hardcode video width and height

Currently, playing a video with :

Width                                    : 1 920 pixels
Height                                   : 1 080 pixels

won't scale correctly because of https://code.videolan.org/jbk/vlc.js/-/blob/incoming/vlc.html#L91 and the vout module does not resize.

Need to be careful about not letting the browser do the resize, and let vlc do it. Cf. https://github.com/emscripten-core/emscripten/blob/main/tests/canvas_animate_resize.cpp#L18