qtmux: for Apple ProRes, allow overriding pixel bit depth for 4:4:4:4 variants

qtmux: for Apple ProRes, allow overriding pixel bit depth for 4:4:4:4 variants

e.g. when exporting an opaque image, yet with alpha channel.

Apple ProRes certification requires that, when a ProRes-writing
application *knows* that the entire frame is opaque, the application
writes only RGB without alpha even when the clip is RGBA. For that,
this tiny change allows the app to override pixel depth when writing ProRes.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/1061>