Alter shader code for soft light to avoid driver bug.

On the n5 there seems to be an issue where negating a variable at the start
of an equation doesn't work. Moving the negated variable to a subtract at the
end works around the issue (as would multiplying by -1.0).

The main bug was caused by the -DaCub in the else if block, but I also
adjusted the else block to make sure the same issue doesn't appear there
at some later date.

BUG=chromium:502434

Review URL: https://codereview.chromium.org/1210573002
1 file changed