Use WELS_ROUND in the neon version of GeneralBilinearAccurateDownsampler

Also use it in the commented out sse2 version - in case it gets
taken into use at some point. (It only works on 32 bit x86
at the moment.)

This makes sure the scaling factors are rounded similarly to the
in GeneralBilinearAccurateDownsampler_c, making sure the
accelerated versions of these function return identical results
to the C version.