Attempt to update Xfermode comments to have parallel structure.

Attempt to update Xfermode comments to have parallel structure.

BUG=none
TEST=none

Committed: https://skia.googlesource.com/skia/+/129319ff55f4be826419ffa04e772e21c6b12400

[Peter Kasting, 2015-10-28 23:33:18]

pkasting@chromium.org changed reviewers:
+ reed@google.com

[Peter Kasting, 2015-10-28 23:33:18]

I found myself struggling to understand how these modes related to each other,
so I made a few small changes:

* For pairs of modes like SrcOver/DstOver, try to write DstOver in the same
order as SrcOver but with all the Ss and Ds swapped appropriately.
* When multiplying color and alpha components, always list color first.
* Factor out (1 - alpha) terms in a few more cases.
* Eliminate "Rc = ".

The resulting descriptions are (for me at least) a little easier to use to
compare one mode to another functionally.

[reed1, 2015-10-29 15:29:40]

The CQ bit was checked by reed@google.com

[reed1, 2015-10-29 15:29:40]

lgtm

[commit-bot: I haz the power, 2015-10-29 15:29:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1406413003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1406413003/1

[commit-bot: I haz the power, 2015-10-29 15:41:18]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/129319ff55f4be826419ffa04e772e21c6b12400