Add split-field interlacing to h265parse & interlace elements
Submitted by Zeeshan Ali
These patches add split-field (alternate) interlacing support to h265parse and interlace elements.
These patches are not yet ready to be merged as the caps lack the needed feature, which will be added soon. The point of submitting them already was to already get opinions/suggestions on the main changes.
Also I'm not entirely sure if the (short-cut) approach I took for interlace is what we would want. Suggestions welcome.