ARM Skia NEON patches - 11 - Blitter_RGB16

Blitter_RGB16: fixes and improvements

- fix alpha calculation: it was still using the old version of
  SkAlpha255To256. 11 more tests pass in gm.

- clean a lot the code: the existing code was "a bit" messy with
  a lot of duplicated hardcoded constants, got rid of all this.

- improve speed a little: part of it as a side-effect of the change
  in the way alpha is calculated but also by grouping loads and stores.

One "issue" was present and still remains: the NEON code doesn't give
the same result as the black blitter on black. It accounts for dozens
of mismatches in gm. Is this considered "not too bad"? Would you be
interested in a NEON version of the black blitter? The current
comments seem to indicate that the black blitter is here only to give
a performance boost when NEON is not presents so I didn't write a NEON
version.

BUG=
R=djsollen@google.com, tomhudson@google.com, reed@google.com

Author: kevin.petit.arm@gmail.com

Review URL: https://chromiumcodereview.appspot.com/18666005

git-svn-id: http://skia.googlecode.com/svn/trunk@10635 2bbb7eff-a529-9590-31e7-b0007b416f81

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