h264parse: Fix framerate calculation and interlaced features

Regarding framerate, the utility function is invalid for figuring out the target framerate. This should solely be based on the VUI timing parameters.

How the individual frames/fields should be represented (ex: repeating fields, or not) should not have an impact on the target framerate.

Fixes framerate caps flipping back/forth with telecine material

Furthermore, we set the proper interlace-mode on the caps since this is the earliest place where we can reliably determine what kind of interlacing (or not) is present. This avoids downstream elements (such as decoders) mis-guessing the interlace mode and flags.