plugin: simplify caps feature selection
plugin: simplify caps feature selection

This patch simplifies the function gst_vaapi_find_preferred_caps_feature().

Instead of intersecting custom caps to find the preferred feature, the peer
caps are traversed in order to find the preferred feature, according to an
ordered feature priority list.

In the case of GLTextureUploadMeta, the colour format is computed using
GstVideoInfo of the selected fixed caps.

https://bugzilla.gnome.org/show_bug.cgi?id=765223

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi