yuv: Remove chroma interpolation, use iterators

[torokati44]

Sadly, this effectively reverts kmeisthax/ruffle#10.
This is technically less "correct", and looks less nice, but in exchange, we gain a whole bunch of speed, the original code simplicity, and most importantly, close similarity to Flash Player.
It also replaces #9, being much-much simpler than that.

To verify, take this test video: testpattern6_h263.zip

(All screenshots are in 5x magnification with "nearest" interpolation.)

This is how it looks in Adobe Flash Player:

Notice how the top vertical red bar is precisely two pixels wide, with no bleeding to the left or right at all.

(The slightly off-shade blue bars above and below the left horizontal red bar are the result of FFmpeg's RGB->YUV conversion. They are there even when I convert the same source image into raw .y4m with yuv420p pixel format.)

This is how it looks in Ruffle now:

And in Ruffle, with this change:

Basically identical to what Flash Player shows, except for some slight noise, most likely from the IDCT.

[torokati44]

I have also not found any H.263 or VP6 video played back by Flash Player where any of the pixels in a 2x2 group had different Cb or Cr values when converted back from RGB according to BT.601.