mft: use NV12 source when the original is not supported

We may try to pick a more fitting format for 10-bit or non-4:2:0 source but that will do for now.

All known encoders accept NV12 as input. They may also accept some form on I420 or RGB32 but we would need a list of favored fallback to test for each encoder we test. So hardcoding NV12 is the safest/easiest for now.