Use proper chroma upsampling for yv12 to yuy2 conversion

The old code did some "averaging" which, while cheap, lead to serious
chroma shift because the weighting factors turned out to be pretty random
(arguably no averaging likely would have been given more correct results).
It also in fact lead to chroma ghosts.
To see why this was wrong read the following and then do the math.
http://www.hometheaterhifi.com/the-dvd-benchmark/179-the-chroma-upsampling-error-and-the-420-interlaced-chroma-problem.html
http://avisynth.org/mediawiki/Sampling

As an example, let's look what happens at line 4 for interlaced content
(where the code would have averaged chroma from chroma line 2 and 4):
Chroma line 2 contains chroma values for line 2 (25%) and 4 (75%) while
chroma line 4 contains chroma values for line 6 (25%) and 8 (75%) of the
original (prior to subsampling) frame.
Average these together and you get something quite wrong. Most importantly
the center of these weights will be at 5.5 instead of 4 (hence chroma shift).
For odd lines it is different (better but still wrong).
So, fix this by using the correct weights for reconstruction of the chroma
values (which is averaging for the progressive case for all pixels since the
samples are defined to be between the lines, and use different weighting
factors for odd/even/"upper"/"lower" lines).

This runs more than twice the instructions (for the mmx case), but I measured
only a performance impact of roughly 5% (on a Athlon64 X2) - seriously bound
by memory access (by comparison the sort-of-pointless post-deinterlace chroma
filter is nearly twice as slow hence if you don't need it because the values
are correct this will be a lot faster).

Note: this is only correct for codecs which use the same chroma positions
as mpeg2 (dv is definitely different, mpeg1 is also different but only for
horizontal positioning, which doesn't matter here). "yv12" as such seems
underspecified wrt chroma positioning.

On another note, while this algorithm may be correct, it is inherently
suboptimal doing this pre-deinterlace (and a post-deinterlace chroma
filter is not going to help much neither except it can blur the mess).
This NEEDS to be part of deinterlace (which btw would also be quite a bit
faster when handling planar directly due to saving one pass of going
through all memory).
The reason is while line 4 will now use the correct weighting factors,
the fact remains it will use chroma values originating from lines 2, 4, 6
and 8 of the original image. However, if the deinterlacer decides to weave
because there is no motion, it CAN and most likely wants to use chroma values
from the other field (hence values originating from line 2, 3, 4, 5 in this
case when using a very simple filter, with appropriate weighting).

--HG--
branch : point-release
extra : rebase_source : 808bb5785ca398970324bea6b391a9e24c576d2f

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