Sk4px: Difference and Exclusion

Sk4px: Difference and Exclusion

This will cause minor (off-by-one) diffs due to a little lost precision:
	colortype_xfermodes
	mixed_xfermodes
	xfermodes2
	xfermodeimagefilter
	xfermodes3
	xfermodes

Desktop:
Xfermode_Difference_aa	9.77ms -> 7.32ms	0.75x
 Xfermode_Exclusion_aa	8.49ms -> 6.21ms	0.73x
   Xfermode_Difference	  17ms -> 7.54ms	0.44x
    Xfermode_Exclusion	13.5ms -> 5.09ms	0.38x

N7:
Xfermode_Difference_aa	32.2ms -> 27.6ms	0.86x
   Xfermode_Difference	43.9ms ->   32ms	0.73x
 Xfermode_Exclusion_aa	40.5ms -> 26.7ms	0.66x
    Xfermode_Exclusion	71.5ms -> 23.9ms	0.33x

This wraps up the xfermodes implemented in Sk4f.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/0135a41e095a433414e21e37b277dab7dcbec373

[mtklein, 2015-05-15 15:47:13]

The CQ bit was checked by mtklein@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-15 15:50:05]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1141213002/80001

[mtklein_C, 2015-05-15 15:54:22]

mtklein@chromium.org changed reviewers:
+ reed@google.com
- mtklein@google.com

[commit-bot: I haz the power, 2015-05-15 16:06:22]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-15 16:06:23]

Dry run: This issue passed the CQ dry run.

[reed1, 2015-05-15 17:31:04]

lgtm

[mtklein, 2015-05-15 17:35:50]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-05-15 17:36:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1141213002/80001

[commit-bot: I haz the power, 2015-05-15 17:36:26]

Committed patchset #5 (id:80001) as
https://skia.googlesource.com/skia/+/0135a41e095a433414e21e37b277dab7dcbec373