#27 Artefacts in 720p encoding

[Thomas Davies]

Lines sometimes appear on smooth areas at the top of the
picture when coding 720p material with -HD720 preset.

The artefacts look like some sort of error in the overlapping
process. HD720 presets have non-square blocks, and so a
possible cause is that x- and y- block parameters are
switched somewhere.

The error may be confined to Intra blocks, but it's hard to
be sure.

Initially, the preset has been changed to use square blocks
to avoid the problem.

[Thomas Davies]

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There's also an artefact at the lower end of the picture, probably
arising from padding being done incorrectly on L1 pictures,
which has a knock-on to L2 pictures.

These problems both go away when SD block sizes are used, so
that's a temporary fix.