vaapipostproc: fix check for compatible src pad capsfilters.

vaapipostproc: fix check for compatible src pad capsfilters.

When an explicit output video format is selected, from an src pad
capsfilter, make sure that the downstream element actually supports
that format. In particular, fix crash with the following pipelines:
... ! vaapipostproc ! video/x-raw,format=XXX ! xvimagesink ; where
XXX is a format not supported by xvimagesink.

While doing so, also reduce the set of src pad filter caps to the
actual set of allowed src pad caps.