colorspace: treat PL_CHROMA_UNKNOWN as _TOP_LEFT

A lot of subsampled content out there is untagged, but should be treated
as _TOP_LEFT content (the de-facto standard chroma subsampling mode).
However, we effectively treat _UNKNOWN as PL_CHROMA_CENTER. To fix this,
make pl_chroma_location_offset explicitly default the chroma location.

Since a lot of users currently just call that function on the chroma
planes always (regardless of subsampling), introduce a new helper
function `pl_image_set_chroma_location` to only set the plane shift for
actually subsampled planes.

Annoying API break, but it is what it is.