vaapidecode: keep src caps and output state in sync

vaapidecode: keep src caps and output state in sync

vaapidecode keeps an output state that use the format
GST_VIDEO_FORMAT_ENCODED, while it crafts a different src caps
for a correct negotiation.

I don't see the rational behind this decoupling, it looks like
unnecessary complexity. This patch simplify this logic keeping
in sync the output state and the src caps.

This patch improves the readability of the function
gst_vaapidecode_update_src_caps() and simplify its logic. Also,
the patch validates if the buffer pool has the configuration for
the GL texture upload meta, in order to set the caps feature
meta:GLTextureUpload. Otherwise, the I420 format is set back.

https://bugzilla.gnome.org/show_bug.cgi?id=744618

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
Signed-off-by: Sreerenj Balachandran <sreerenj.balachandran@intel.com>