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:

Review URL: https://codereview.chromium.org/1202013002

1 files changed, 4 insertions, 2 deletions

diff --git a/src/opts/Sk4px_none.h b/src/opts/Sk4px_none.h
index d3ead31491..ce2f8452e5 100644
--- a/src/opts/Sk4px_none.h
+++ b/src/opts/Sk4px_none.h
@@ -11,8 +11,10 @@ namespace { // See Sk4px.h
 
 static_assert(sizeof(Sk4px) == 16, "This file uses memcpy / sk_memset32, so exact size matters.");
 
-inline Sk4px::Sk4px(SkPMColor px) {
-    sk_memset32((uint32_t*)this, px, 4);
+inline Sk4px Sk4px::DupPMColor(SkPMColor px) {
+    Sk4px px4 = Sk16b();
+    sk_memset32((uint32_t*)&px4, px, 4);
+    return px4;
 }
 
 inline Sk4px Sk4px::Load4(const SkPMColor px[4]) {