playbin2: Make sure that the decoders we plug are compatible with the fixed sink

playbin2: Make sure that the decoders we plug are compatible with the fixed sink

The fact that a decoder is not compatible with the fixed sink
is currently happenning in the case where we have hardware accelerated
video decoders on the system (especially vaapi elements that are actually plugged),
and the user is providing a sink that doesn't support the surface.

A simple example that shows how it used to crash on a system where gstreamer-vaapi
is installed:
    gst-launch playbin2 video-sink=xvimagesink uri=/codec/supported/by/vaapi

What we are now doing in this case, is avoid using the accelerated
decoder and plug a "normal" decoder instead (if avalaible).

This commit doesn't handle the case where we have hardware accelerated
demuxing.