The numeric imprecision can accumulate quickly and cause visible
sampling errors.

Bug chromium:848238

Change-Id: Ie41c10a0462e5b5e5c3ed5f7329ece00dab3f7f9
Reviewed-on: https://swiftshader-review.googlesource.com/19148Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>