common: cleanup
common/videolink: cleanup/minor tweaks

use ptrdiff prevline offset in hqx filters to facilitate use of lea.

use endptr rathern than loop ctr in rgb2yuv to avoid gcc/x86 rmw
spill.

avoid reloading rhs input pixel after storing lhs output in rgb2yuv
(no restrict keyword in c++98).

fix hypothetical component overflow in hq2x blend9.
common/resample: cleanup

rename polyphaseconvoluter to polyphasefir.

fix broken upsampler zero stuffing (was never tested, surprisingly the
surrounding code appears to work fine).