compositor: expose zero-size-is-unscaled property

compositor: expose zero-size-is-unscaled property

When that property is left to its default, the width and height
property considers frames from input pads with width or height <= 0
should be left unscaled in that dimension.

Setting this property to FALSE 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 default value of the width and height properties is set to -1, so that
the default behaviour stays consistent whether that new property is set
or not.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/923>