Alter shader code for soft light to avoid driver bug.

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
Committed: https://skia.googlesource.com/skia/+/724ae2823e1481e162b576ecfe8525a329c100d5

[egdaniel, 2015-06-24 14:51:52]

egdaniel@google.com changed reviewers:
+ bsalomon@google.com

[bsalomon, 2015-06-24 14:56:55]

On 2015/06/24 14:51:53, egdaniel wrote:

lgtm

[egdaniel, 2015-06-24 15:01:06]

The CQ bit was checked by egdaniel@google.com

[commit-bot: I haz the power, 2015-06-24 15:01:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1210573002/1

[commit-bot: I haz the power, 2015-06-24 15:14:58]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/724ae2823e1481e162b576ecfe8525a329c100d5