filters: ResizeLinearLF: More work

Removes useless if in inner cycle which brings another 10% speedup.

Do not fetch border rows twice about 20% speedup.

Get rid of the C macro, use jinja macro.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: ResizeLinearLF: Improve precision

Improves precision which fixes artefacts when downscaling from big image
to small image or with close to 1:1 ratio.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>