video-converter: Add fastpaths from I420/YV12, YUY2, Y42B and UYVY to v210

Adds fast paths from I420/YV12, YUY2, Y42B and UYVY to v210 (essentially a reverse of this MR). Also cleans up the v210 packing code to match what's being done in the fast-paths, making it correctly zero-out unused component values.

Fixes a bug with the v210 <-> I420 fast path where the last line, which was handled separately from the rest of the image, would not be converted correctly if the height was an odd number - this was caused by not handling the difference between the size (8bit vs. 16bit) of data expected by I420 and v210 (un)pack functions correctly.