Alongside the basic/trivial deinterlacing modes, this contains an
implementation of the raw yadif deinterlacing kernel, based on
vf_yadif_cuda.cu by @philipl. Currently only supports fragment shaders,
with no compute shader speedup (although such a thing is conceptually
possible, to share duplicate sampling work between adjacent pixels).

Design the API in such a way that it permits field-merging (PAFF) should
the need should arise in the future. Also adds a few related helpers.

Closes: videolan/libplacebo#20