WIP: videoscaler: Don't vertically shift both fields away from each other when doing interlaced scaling

Previously we would shift the top field 0.5 * out_height / in_height lines down and the bottom field that amount up.

This seems to have no justification at all: while the top field is one line above the bottom field this has no meaning at all for rescaling. Each field is scaled independent from the other field to the target field size and then both fields are interleaved again.

If we shift both fields then this causes visual defects on the output, with lines of both fields being visibly offset from each other, and when deinterlacing causes the whole output to be blurred much more than necessary.