pngenc: make setcaps more robust, use gstvideo functions

pngenc: make setcaps more robust, use gstvideo functions

A setcaps function needs to actually verify the caps carefully. In this case,
it was possible to e.g. link a video decoder with YUV+RGB template caps to
pngenc.  That would cause a crash when the decoder pushes a YUV buffer. Same
thing when pushing a valid buffer that exceeds the resolution limits.

Also, missing framerate caps field would cause a glib critical warning due to
invalid GValue. This fails hard now.