Remove luma width check in fguv_32x32xn

This would affect the output in samples with an odd width and horizontal
chroma subsampling. The check does not exist in libaom, and might cause
mismatches.

This causes issues in the sample from #210, which uses super-resolution
and has odd width. To work around this, make super-resolution's resize()
always write an even number of pixels. This should not interfere with
SIMD in the future.