Implement deinterlacing

This feature roughly consists of two separate aspects:

1. Adding an API to represent interlaced content in the renderer. Maybe tie this into the interpolation API somehow? Since interlacing is a fundamentally temporal thing - but unlike interpolation, it needs to be done on the opposite end. Tricky. If this turns out to be too involved, we could just regress to a single-field only representation, which would just require a change to pl_plane / pl_image.
2. Implement the corresponding deinterlacing shaders. Ideally, we want something high quality here - but even a simple bob deinterlacer wouldn't be useless.