gtkmediafile: Only unmap the GstVideoFrame in the GBytes destroy notify

gtkmediafile: Only unmap the GstVideoFrame in the GBytes destroy notify

The memory pointed to by GstVideoFrame::plane_data becomes invalid after
unmapping causing the GBytes to point at some random memory if the
unmapping is not deferred until its destroy notify.

When the GStreamer buffer is backed by normal system memory this is not
a problem but if it is backed by e.g. an OpenGL texture, dmabuf or some
other hardware-specific memory this will otherwise cause interesting
problems.