Speed up resampling a bit again

[homm]

Not a big deal, but why not )

Scale 2560×1600 RGB image
    to 2048x1280 bil    71.30 Mpx/s    77.93 Mpx/s
    to 2048x1280 bic    50.37 Mpx/s    54.74 Mpx/s
    to 2048x1280 lzs    37.40 Mpx/s    39.01 Mpx/s
    to 5478x3424 bil    15.03 Mpx/s    16.56 Mpx/s
    to 5478x3424 bic    11.63 Mpx/s    12.50 Mpx/s
    to 5478x3424 lzs     9.27 Mpx/s     9.84 Mpx/s

[homm]

Absolute coverage decreased by -0.01% but relative coverage increased by +18.31% compared to bbe07d4

Looks like codecov is smarter than coveralls

[homm]

@wiredfool if you have a couple of minutes, I'd like to ask you to look at this macros at first. The same macros are used in many recent PR's. I'm not very familiar with endings.

[wiredfool]

LGTM. I've fired up the antique bigendian PPC machine I have here, and the only failures I'm seeing are with 16bit jpeg2k files, but those tests appear to fail all the way back.

These macros appear to be fine, assuming that we've got enough testing on this part.