Martin Storsjö authored May 13, 2021 and Jean-Baptiste Kempf committed May 13, 2021

In 16 bpc, the pixels are 16 bit integers, but valid pixels only
are up to 12 bits, and the scaling buffer only contains 4096
elements.

The src pixels are, normally, supposed to be valid pixels, but when
processing blocks of 32 pixels at a time, it can operate on
uninitialized pixels past the right edge.

Before:               Cortex A53      A72      A73  Apple M1
fgy_32x32xn_16bpc_neon:  10372.5   8194.4   8612.1  24.2
After:
fgy_32x32xn_16bpc_neon:  10837.9   8469.5   8885.1   24.6