affine-bench: remove 8e margin from COVER area

affine-bench: remove 8e margin from COVER area

Patch "Remove the 8e extra safety margin in COVER_CLIP analysis" reduced
the required image area for setting the COVER flags in
pixman.c:analyze_extent(). Do the same reduction in affine-bench.

Leaving the old calculations in place would be very confusing for anyone
reading the code.

Also add a comment that explains how affine-bench wants to hit the COVER
paths. This explains why the intricate extent calculations are copied
from pixman.c.

[Pekka: split patch, change comments, write commit message]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ben Avison <bavison@riscosopen.org>