Update some Sk4px APIs.

Update some Sk4px APIs.

Mostly this is about ergonomics, making it easier to do good operations and hard / impossible to do bad ones.

- SkAlpha / SkPMColor constructors become static factories.
- Remove div255TruncNarrow(), rename div255RoundNarrow() to div255().  In practice we always want to round, and the narrowing to 8-bit is contextually obvious.
- Rename fastMulDiv255Round() approxMulDiv255() to stress it's approximate-ness over its speed.  Drop Round for the same reason as above... we should always round.
- Add operator overloads so we don't have to keep throwing in seemingly-random Sk4px() or Sk4px::Wide() casts.
- use operator*() for 8-bit x 8-bit -> 16-bit math.  It's always what we want, and there's generally no 8x8->8 alternative.
- MapFoo can take a const Func&.  Don't think it makes a big difference, but nice to do.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/059ac00446404506a46cd303db15239c7aae49d5

[mtklein, 2015-06-22 17:12:36]

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

[commit-bot: I haz the power, 2015-06-22 17:12:46]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1202013002/1

[commit-bot: I haz the power, 2015-06-22 17:14:00]

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

[commit-bot: I haz the power, 2015-06-22 17:14:01]

Dry run: Try jobs failed on following builders:
  Build-Ubuntu-GCC-Mips-Debug-Android-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu-GCC-Mip...)

[mtklein, 2015-06-22 17:15:10]

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

[commit-bot: I haz the power, 2015-06-22 17:15:21]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1202013002/20001

[mtklein, 2015-06-22 17:23:45]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2015-06-22 17:23:52]

The CQ bit was checked by mtklein@google.com

[mtklein, 2015-06-22 17:23:53]

lgtm

[commit-bot: I haz the power, 2015-06-22 17:24:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1202013002/20001

[commit-bot: I haz the power, 2015-06-22 17:39:43]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/059ac00446404506a46cd303db15239c7aae49d5