When using chroma subsampling, the warp filter currently behaves
strangely when projecting chroma pixels, especially when the
subsamplings are not equal along the x and y axes.

For example, when subsampling_x = 1 and subsampling_y = 0, we
calculate the destination coordinates (dx, dy) from the source
coordinates (sx, sy) as:
dx = project(2*sx+0.5, 2*sy+0.5)/2 - 0.5
dy = project(sx, sy)
where project() applies the affine warp model.

This patch changes to a simpler and more consistent model,
where we:
* Project the chroma sample into luma coordinates, taking
  the chroma sample to be co-located with the top-left luma
  sample in its (2x2, or 2x1, or 1x2) subsampling block
  (this is done for simplicity; we don't expect the exact
   position to make much difference to the output quality)
* Apply the transformation in luma coordinates
* Project the resulting luma sample back into chroma coordinates

Change to software speed is in the noise, but this approach
should be simpler in hardware, and should slightly improve
quality for 4:2:2 and 4:4:0 videos.

Change-Id: Idd455fdd3897594ca7d4edff5b85b78961d1638d