The value of the mirrored x coordinate is almost exactly integer
all the time. On 64-bit systems the CPU converts it into integer
correctly, but on 32-bit systems it probably doesn't download it
from the FPU register (which are 80-bit and therefore more precise),
so the value got truncated to the decremented value. It results in the
lines becoming shaky due to random precision errors.

Now we explicitly round "almost-integer" part of the sum so no precision
fluctuations happen.

BUG:341577