compositor: use -1 as the value for keeping input pads unscaled

Previously, the width and height property considered frames from input pads with width or height <= 0 should be left unscaled in that dimension.

This patch changes that behaviour to < 0, as when animating these properties, 0 should be a valid end value (eg. shrinking an input stream until it disappears).

The assumption here is that nobody actually uses 0 explicitly to require the original size, it was not documented as such, simply the way the default value was interpreted. We simply make the default value -1 and move on.